Though only introduced in the third season of Star Trek: The Original Series, the Klingon D7-class battle cruiser, despite its limited number of appearances, captured the imagination of the audiences, and went on to become one of the most signature ship designs of the franchise. Ironically, the original filming model was not commissioned by the producers for the show who had to contend with severe budget cuts during the third season, but resulted first and foremost from the wish of kit producer Aluminum Model Toys (AMT) to do a follow-up of their highly successful USS Enterprise model kit, as its designer revealed in Star Trek: The Magazine Volume 2, Issue 9, p. 66.
The D7-class studio model was originally designed by Matt Jefferies with initial input from friend and AMT account manager Stephen Edward Poe, and he needed about two months from start to final design sketch. Its design was explored in the Star Trek: The Original Series Sketchbook where it was explained that in Jefferies' attempt to create the D7, he "had to design a ship that would be instantly recognizable as an enemy ship, especially for a flash cut. There had to be no way it could be mistaken for our guys. It had to look threatening, even vicious." Taking an aerodynamic approach to his design, he ultimately "modeled it on a manta ray, both shape and color, and that's why it looks as it does in the original series." In a contemporary 1968 interview, Jefferies has delved somewhat deeper into his thought processes behind the design, having stated, "We had already established the essential character of the Klingons, so we had really more to draw on in background than we originally had on the Enterprise. The Klingon character was different and clearly defined in several scripts. We tried to keep some of that character in the design of the ship – cold and, in a sense, vicious. We tried to get into it some of the qualities of a manta ray, shark, or bird of prey, because the Klingons follow that general feeling. Another requirement was that we had to get a feeling their ships were on a par with the starships in equipment, power, size etc. After many, many sketches and many evenings, it finally evolved. Everyone liked it , and that's what we built. It was strictly an extra-curricular activity on my part." (Inside Star Trek, issue 4, pp. 3-4)
Unable to divulge the AMT connection at the time, though he evidently could not refrain from making the closing remark, which hinted at the connection, Jefferies, no longer under the purview of franchise policies, much later revealed on his design work,
"AMT didn't have any design input whatsoever, and by that point Gene pretty much left me on my own. I designed it here at home, because there was neither the time nor the money allowance to do it at the studio. Naturally, I thought it had to look as far out as we thought the Enterprise did. I was after a shape and didn't really know what the shape should be. I started doing little sketches, trying to come up with something; God knows how many there were. I saved some of them, but I'm sure I must have ashcanned maybe a hundred balled-up pieces of paper. It's like when you make a mistake in arithmetic and you go back over the same piece of paper and keep making the same dumb mistake; you've got to throw it away and start from scratch.(...) "The Klingons were supposed to a pretty wicked people, so I wanted something with a "killer potential" that would look wicked. Basically, I was feeding on the look of the stingray, or the manta ray, for part of the shape. Even though it is not dangerous, I think a lot of people think the manta ray has a very vicious look to it, yet when it swims it is very graceful. I was trying to get all of that in there. Then the coloration came directly from a shark, it's a grayish-green on top and a lighter gray underneath." (Star Trek: The Magazine Volume 2, Issue 9, pp. 66-69)
Having decided early on to make use of the same basic elements as with the Enterprise, twin nacelles and separate engineering and command hulls, with which he continued to experiment in layout and configuration, he added, "Sometimes if you feel you have something, which could be kind of rare, you turn it around in as many ways as possible, and all of a sudden something may pop up that makes more sense." The ship's design was perfected by the twenty-fourth sketch on 20 November 1967.
Jefferies sold off his original design sketches, as well as the below mentioned engineering drawings, on 12 December 2001 in the Profiles in History The Star Trek Auction, in order to raise funds for the charitable organization Motion Picture and Television Fund. Prior to the auction, most of his D7 design art, several of which previously unseen, was published in Star Trek: The Original Series Sketchbook, and in the one month later (but earlier conducted) interview issue of Star Trek: The Magazine.
The original filming model
Jefferies proceeded to draw up detailed engineering drawings which also specified the scale in relation to the Enterprise, and took those to AMT. The scale comparison drawings were also used as templates for the back-lit computer console readouts seen in the episode "The Enterprise Incident". Under Jefferies' own personal supervision, Gene Winfield's AMT-operated "Speed & Custom Shop" proceeded to manufacture two three-foot long "master tooling" models (more precisely, the dimensions were 28 × 20 × 7 inches). A master tooling model served as a template for the molds from which AMT would cast the parts for their model kits. Jefferies recalled, "The master models were quite large; probably close to 18 inches across, I guess. They used what they call a pentagraph [sic]; at one end there was a stylus that traced its way over the master model, and at the other end there was a tool that carved out the same shape in tooling steel, which became the mold they built the kit from. [remark: the mold for the kits were pantographed at half-scale of the master model] I was there at about 2 o'clock in the morning when they ran the first two or three through the machine. They weren't perfect, so they said "We'll take out a fraction here, and a fraction here." Then they'd run two or three more. If I remember correctly, it was about 10 o'clock when the first one came out that they said was perfect. They ran maybe another half a dozen, and checked those out. One of which I still have; the box has never been opened. Then the machine was put in operation, and after that one came out every 20 seconds." (Star Trek: The Magazine Volume 2, Issue 9, pp. 69-70)
Under their exclusivity agreement the strapped-for-cash studio immediately appropriated one of the tooling master models for filming purposes. The model was subsequently turned over to the Howard Anderson Company for final detailing and filming stock footage for use in the show. In order to make maximum use of their new nifty model and as a courtesy to AMT (who after all paid for the model), in order to get them the most exposure for their new "Klingon Battle Cruiser" model kit (no. S952, the D7-class designation did not yet exist at that time), the producers decided to use it wherever possible in the remainder of the third season of TOS and so the new model first appeared onscreen as a Romulan battle cruiser in "The Enterprise Incident", although it was actually first shot as a Klingon battle cruiser for "Elaan of Troyius" (which aired later). Its origin as a master tooling model for a model kit, as opposed to being an actual filming model was evident in the fact that neither model, both constructed out of solid wood, had internal lighting.
Incidentally, Poe already incorporated Jefferies' scale drawings – slated to appear on the sides of the corresponding model kit box for its intended 1968 holidays season release – as illustrations in his reference book The Making of Star Trek (pp. 184-186), even though the ship itself had yet to premier on screen in "The Enterprise Incident" later that month, when the book was published at the start of September 1968. Rather ironically, by not mentioning the AMT connection, it was Poe himself who actually started the four decades long myth that the D7 was a studio initiative which it was not, and who had not commissioned it, despite assertions and assumptions by numerous reference authors, fans and the franchise itself to the contrary afterwards. It was not until 2002 that Jefferies had set the record straight.
The original studio model was originally finished at the Howard Anderson Company in a two-tone paint scheme (light green underneath and gray on top), applied personally by Jefferies. Jefferies also designed and applied the Klingon emblem and the Klingon lettering – also originally intended for the model kit – , the first time ever either one appeared in the Star Trek franchise. But the color scheme was obscured by studio lighting conditions, combined with the effect of lighting bouncing of the blue-screen onto the model, an effect known as "blue spill", resulting in what appeared to be a blue-gray overall color on screen. This impression was reinforced by the box cover art of the AMT D7 model kit release. Interestingly, in the Animated Series episode "More Tribbles, More Troubles", Koloth's battle cruiser, IKS Devisor, adheres to the original paint scheme, though the two colors are reversed. Silver detail sections of the model were an adhesive Mylar.
After The Original Series was canceled, the original model of the D7 was, hand-delivered by Dorothy Fontana on 7 November 1973, donated to the Smithsonian National Air and Space Museum (NASM) along with the tiny "Catspaw" model of the USS Enterprise. "I was trying to figure out how to pack it up to ship it. Dorothy Fontana was headed back to D.C. and agreed to take it for me; we put it in a plastic garbage back, which was not deep enough to to take the whole thing, so the head of it stuck out. Somebody on the airliner recognized it, so they unwrapped it and it toured the airliner!", Jefferies recalled. (Star Trek: The Magazine Volume 2, Issue 9, pp. 69-70; Star Trek Giant Poster Book, issue 10, 1977)
Star Trek: Phase II derivative
In the latest script treatments of the pilot episode "In Thy Image" of the projected Star Trek: Phase II television project, the Klingon-V'Ger encounter was already foreseen for that production. To this end, the original filming model was returned on loan to Paramount Pictures in the last quarter of 1977 for use on the Phase II television production, and where it received a new, three-tone, paint job (gray, dark green and metal blue). It is conceivable that the new paint job was applied to repair damage, as the model was used as a master to take molds from in order to cast copies for intended use in the production. At the time, Jim Dow of Magicam Inc., whose company was entrusted with the visual effects (VFX) for Phase II, recalled when it was later decided to build a larger model for the movie project, "We began with the Klingon spaceship, as we had the 18-inch model from the original television series which had been loaned back to the production by the National Air and Space Museum. The basic form remaining similar, we began to make patterns and take measurements of the original, and a set of drawings was produced and blown up to eight feet then dropped back to four feet when [Robert] Abel [& Associates] [RA&A] decided they would never be able to get far enough away from the model to photograph it." (American Cinematographer, February 1980, p. 153) To handle the original studio model, Dow actually sub-contracted Gregory Jein, for his very first professional, albeit uncredited, assignment on the Star Trek franchise. Jein, who at the time was rapidly making a name for himself, having been one of the chief model makers on Close Encounters of the Third Kind, disassembled the model and used the parts as masters to obtain molds from them. From these he cast fiberglass parts for the construction of a "hero" model. At that point in time, as indicated in early script treatises of "In Thy Image", the Klingon vessel belonged to the heavy cruiser Koro-class. (Star Trek Phase II: The Lost Series, p. 64)
However, it was quickly decided that the model would not stand up to the requirements television standards of the late 1970s demanded, and especially not to the big screen requirements after Phase II was upgraded to a major movie project in October 1977. After the early January 1978 inspection of the Phase II models it was definitively decided to discard all the models, including Jein's model, and start all over again. (Star Trek: Creating the Enterprise, 1st ed, p. 46; Return to Tomorrow - The Filming of Star Trek: The Motion Picture, pp. 71-72)
After the decision was made that three-foot model was not up to the task for the movie-upgrade, the original television studio model was of no further use, and so it was send back to the Smithsonian, where it sat in an unlit display case for the next ten or eleven years at the NASM's Suitland, Maryland Garber Preservation, Restoration and Storage Facility, not open to the public, before being disassembled and put away in storage. Before its disassembly it was photographed by studio model aficionado William S. McCullars for an article he wrote on the model that was published in Starlog, issue 147. It was still in its new, reapplied paint scheme when Ed Miarecki undertook a major renovation on the model in 1991 in preparation for the Star Trek Smithsonian Exhibit, and in 1992 the D7 model and the K't'inga-class model, which it had spawned, were reunited for a short time when the K'tinga was on loan to the Smithsonian.  Miarecki, apparently unaware of the original paint scheme at the time, repainted the model in the blue-gray mono-colored scheme as was normally perceived by television audiences.  Having only been displayed publicly once, the model, though in storage, is still in the possession of the Smithsonian.
Greg Jein saved his model from the dumpster and retained both the unused hero model and the molds from which he had constructed it. He finished up on the model and it has been featured in the reference book The Art of Star Trek, page 19. Jein later gave the model to Doug Drexler, who had it in his possession for several years. Drexler eventually auctioned the model off as Lot 13 in the August 8th, 2010 Propworx The official STAR TREK prop and costume auction, estimated at US$2,000–$3,000, where it sold for US$12,000.
The second "master tooling" model
The second master tooling model was handed over to the studio a short time later after AMT was done with it. It received the same finishing touch as the first model and was at a casual glance nearly indistinguishable from the first one (exactly two times the size of the AMT model kit). There were, however, subtle differences between the two. The bulk of the differences were located on the engine nacelles. The position of the engine recessed side detail was moved more forward on the AMT prototype, in relation to the pylon foil vent. The detail piece on the engine bottom was a little further back on the studio model – about a third of its length back. The AMT model lacked both the feature in the torpedo launcher mouth, as well as the grills in the recessed forward "wing intakes" that were present on the screen-used model. And finally, and most notably, the screen-used model version had two sets of two raised, flat chrome strips, whereas the AMT version sported a raised strip intersecting the chrome part of the side engine; the chrome part on the AMT prototype was a single strip shape that had a half-round profile view, and as such it was transferred onto the molds of the model kit, in effect reflecting Jefferies' original design intent.
In order to have it correspond to the screen version, AMT corrected this for their 2011 model kit re-issue (no. AMT699/720), as it from then on featured the two strips. Ironically, from its very first release onward, the AMT model kit version was represented by the screen version on the box art of the kit, as it featured an edited photograph of the model as finished at Anderson's.
Although slated to be also used for filming it was never used as such and ended up in Roddenberry's office for awhile. Roddenberry gave the model away to Stephen Edward Poe in recognition of his help in establishing the co-operation between the Desilu studios and AMT (in whose employ Poe was at the time). It changed hands several times after that and was between 1998 and 2006 offered up for auction no less than four times. The first time it appeared, it was offered up for auction by Poe at Christie's Film and Television auction of 18 June 1998 as Lot 71 with an estimate of US$15,000–$20,000, selling for US$11,500 (including buyer's premium), and was immediately offered up for auction afterwards as Lot 245 in Profiles in History's Hollywood Memorabilia Auction 5 on 12 December 1998, selling for US$35,000. The third time it appeared at auction was on 31 March 2003 in Profiles' Hollywood Auction 18 with an estimate of US$60,000–$80,000, selling for US$55,000 ($64,900 including buyer's premium), before finally auctioned off as Lot 311 in Profiles' Hollywood Auction 24 on 31 March 2006 for US$65,000, estimated at US$65,000–$85,000.   It should be noted that all auction descriptions contained inaccuracies, all erroneously claiming that it was a screen-used model.
The model was on the last occasion acquired by Microsoft's co-founder Paul Allen for his Science Fiction Museum and Hall of Fame in Seattle, now known as the Museum of Pop Culture (MoPop),  where it currently resides.  The deal was brokered before auction start and the model was therefore not featured in the auction itself. Yet, as that model still carries its original paint scheme, it serves as an useful reminder of what the screen-used model had actually looked like originally. After an absence of several years, the model went on public display again in the Star Trek: Exploring New Worlds exhibition at MoPop from 21 May 2016 to 28 May 2018  – it too alongside its K't'inga-class offspring, in the meantime acquired by Allen as well in the 40 Years of Star Trek: The Collection auction, held the same year – , which subsequently became a touring exhibition.  Incidentally, Allen had also acquired several pieces of Jefferies' design art, including the two above-featured scale comparison charts, which were also featured in the exhibit.  
The Motion Picture D7 aka Koro-class model
When the Phase II project was upgraded to a major movie project, what ultimately was to become Star Trek: The Motion Picture, the decision was made to have Magicam (which, while pulled from the VFX production, was retained as the studio model vendor), no doubt due to their by then familiarity with the design of the ship, and supervised by Dow, built a new, more detailed twice as large scale model of the D7, based on the molds taken from the by now-disassembled original television model. Began at the start of February 1978, the D7/Koro-class model was the first model Magicam tackled for the upgrade, after the company received the blueprints from RA&A's Art Director Richard Taylor, specifying its new movie dimensions. Taylor specified the model to be 54 inches long, 40 inches wide, and 12 inches high, with a scale of 1 inch = 10 feet.  Also referring to the Phase II predecessor, Taylor clarified, "(...) that was built before we got involved. I redesigned all the surface textures, the photon torpedo tube and many other details. I tried to put a kind of bird-feather design on the surface. One of the things we did with all of the models was to give their surfaces details and interesting designs. A smooth object has no scale so it's important in model work to find ways of creating scale. Sometimes it's very subtle but it's one of the most important elements in model photography. We did re-build the Klingon ships and that was one of the first things we finished, actually," 
The early start was facilitated by the fact that the basic design of the model was to remain unchanged. Chris Ross was the appointed lead modeler working on the model. The most noticeable difference from the original filming model, aside from its size, was the application of more subtle hull paneling, reminiscent of the model Greg Jein later built for Star Trek: Deep Space Nine. Despite their familiarity with the design, a construction error crept into their build as noted by Production Illustrator Andrew Probert of Astra Image Corporation (Astra), the art department subsidiary of RA&A which had taken over the VFX work from Magicam. Probert observed, "When( I first visited their shop and saw a partially built Klingon cruiser, it was apparent to me that its neck-piece was crooked. As part of my newly-assigned responsibilities, I brought this to their attention. The design of the cruiser, with its varying proportions and angles, was such that no one else had noticed it and when I came out of the blue with my observation it didn't go over too well. They told me that it would cost too much time and money to correct. As it turned out, at a much later time in the production, the need arose for a structural change, so the neck was realigned at that time. From that time onward, nothing in the way of design or drawing submitted by me was ever acceptable to Magicam." (Fantastic Films, March 1980)
Though commonly known as the "6-foot model", the exact measurements of the new, twice as large model, turned out to be 47½ × 35 inches.  Construction on this model,  outfitted with an internal lighting system, and subsequent test shooting were in progress and signed off on when Magicam delivered the model – , the first studio model started and completed by the company for the movie incarnation – to Astra, located on Seward Street, Los Angeles, responsible for RA&A's studio model effects photography, in July 1978. (Return to Tomorrow - The Filming of Star Trek: The Motion Picture. pp. 81, 158, 326 & 386)
Yet, upon delivery in July, and after having seen the lighting test footage, Taylor decided that further refinements were necessary and reverted the model back to Magicam/Astra for upgrades. He deemed the model not detailed enough and thus unsuitable to meet full theatrical feature movie requirements, therefore needing the additional detailing he requested. He elaborated, "We also detailed out the Klingon ships adding detail to the bodies, the nose pod, the photon torpedo tube, and the bridge. I designed a new bird-like paint design for the wings and body of the Battle Cruisers. (...) Michael Sterling and I designed a new Star Fleet typeface, a Klingon typeface, and new logos for the Star Fleet and the Klingons." (Star Trek: Creating the Enterprise, p. 104) One of Magicam's modelers who was assigned this task was its only female employee at the time, Zuzana Swansea, who had also been part of Ross' original construction team.   Production Illustrator Andrew Probert further elaborated on the rework done on the model at that point in time,
"Due to continuing miscommunication with Magicam, the Klingon model eventually came to our Seward shop for additional detailing. We attempted to stay as close to the TV series version as possible, but after running some camera tests we discovered that an additional level of detail was needed for widescreen photography. A series of slides was taken of the model. We had photostats made of them and I did some initial design work on top of the stats. We painted it a darker color. I've always liked the idea of a black spaceship – which would have been impractical for this movie; so we came up with a dark military green. I had an idea that was carried out beautifully by Ron Gress. Ron painted a giant Klingon symbol on the underbelly of the ship. It was a revision of the old Klingon symbol." (Starlog, No.32, March 1980, p. 63)
How pressured for time the studio was by then, was experienced by Magicam's Model Electronics Engineer Paul Turner, who was responsible for the lighting rig of the model,
"They [remark: RA&A] decided they wanted some additional work done on the model and they convinced Paramount that they should spend additional time and money, and it was returned to us. We added some lights to it; we changed the structure just a little bit [remark: the neck piece of Probert's observation]; we did quite a number of things. Everybody worked together around the clock to get the thing out. Then we has an unveiling where people were called at Paramount, and various interested parties, to coma and see the Klingon. They were working on finishing the detail right up until the time the executives were due to arrive. Twenty minutes before the executives were due to arrive they were still in there painting and spraying and touching up on the detail. I hadn't been able to get in there and hook up the wiring yet. I finally got my chance to work on the thing and then everyone came into the room and started looking at it, which made it very difficult for me to concentrate on what I was doing, so I threw everybody out. I said, "Out please. Everybody leave. Give me fifteen minutes to hook the system up." And I threw out everybody, executives and everybody. I got it hooked up and got everything working. Fortunately everything worked. I had pretested all the sub-assemblies, but you never knew when something is going to go wrong. Fortunately everything worked and everyone could come back in and see the model, which was about four feet [sic.] long." (Enterprise Incidents, issue 13, p. 24)
However, even further refinement for movie purposes was deemed necessary at a somewhat later time the subsequent year, which was lastly done at John Dykstra's Apogee, Inc. model shop, resulting in this model eventually morphing into the K't'inga-class studio model. (Cinefex, issue 2, p. 52)
Already envisioned for Phase II, the outcome of the Klingon encounter with V'ger was originally story-boarded as ending with the destruction of the Klingon vessels, and that concept was initially carried over to its movie successor, visualized in concept art by Mike Minor.
To this end breakaway models were constructed at Magicam for their on-screen destruction, using Jein's molds, deemed satisfactory this time around to this particular end. As shown in the contemporary documentary, The Making of Star Trek: The Motion Picture, at least twelve three-foot breakaway models were constructed for the sequence, produced from Jein's molds taken of the original studio model. The documentary also divulged that effects footage of the destruction was actually already shot at Astra's filming facilities in late 1978.
Magicam shop supervisor Jim Dow has clarified,
"We also built an 18-inch [sic.] explodable Klingon cruiser model, which went through a great deal of testing at Astra. We made molds so that we could produce many, many shells for them to experiment with in a variety of materials. They were trying to create an implosion effect. But all we were asked to do at that point was to build the explodable modles, and we were not asked to get involved in the actual shooting of them, or even devise ways of shooting them. So I made suggestions, and pulled back and built the models.
"Joe Viskocil experimented with a lot of ways of imploding them. He was going to explode them and then Abel was going to do some photographic tricks. Richard Taylor's concept for the experiments was to explode the Klingons, creating a ball of gases, and then implode that.
"I think, in actuallity, those implodable Klingon models never made it to the screen because the final Dykstra shots of the Klingons being destroyed were done as an optical with the electronics effect of Larry Albright spears. I can't really be positive that was eactly what was done to create thet effect, but Larry is an electronics experimenter that works around town with everybody, including Dykstra, Doug Trumbull and us." (Return to Tomorrow - The Filming of Star Trek: The Motion Picture, p. 290)
Mistaken about the Albright technology (see: main article), but right about the newly designed opticals, Dow was referring to Dyktra's company Apogee, Inc. when they came aboard in March 1979, having been made responsible for the encounter sequence after RA&A was pulled from the project, an entirely different demise sequence was conceived and produced by Apogee, leaving the original demise concept and its filmed footage thereof discarded. The by Dow outlined imploding sequence was described ad verbum as such in the final shooting script as scene 25 – thus dutifully adhered to by Viskocil and Abel when creating the effect – and still in place when Dykstra altered it.
Concept artist Minor was glad that the original destruction concept had gone, stating, "The Abel group was forever causing problems. Mass sequences that they had designed which were tested and shot will never been seen; and rightfully so, if you ask me," also referring to the ill-advised hiring of the company for The Motion Picture. (Return to Tomorrow - The Filming of Star Trek: The Motion Picture, p. 290) Dow himself was not rueful either that his models were eventually not featured in the film when he, referring to the immense time pressure Dykstra and his company were under at the time, commented, "When I saw the Klingon destruction in the film, I didn't miss the implosion concept at all. I thought that John did an admirable job, based on the time that he had to shoot the sequence, his equipment, and so on – all of the extreneous things. If someone had come to us and said, you know, "You've got three months to make this piece of film and we're going to give you the model," and so on and so forth, I'd be scared out of my boots. But he did it. He pulled it off. You can sit around and you can tear it apart, but I don't think it's necessary. I think, given all of the parameters, he did an excellent job. And it certainly gets a big reaction from the audiences in the theaters." (Return to Tomorrow - The Filming of Star Trek: The Motion Picture, p. 444)
The "Trials and Tribble-ations" model
In 1996, a new model of the D7-class, IKS Gr'oth, was created by Greg Jein for Star Trek: Deep Space Nine's episode "Trials and Tribble-ations". While referenced, but not seen, in the original version of the "Trouble with Tribbles," the model featured an amalgam of detail from both the Original Series model and the K't'inga-class seen in Star Trek: The Motion Picture. The new model bore a pale green coloration, in line with future Klingon vessels. Jein, a passionate life-long fan of the original series, used his set of molds of the original Star Trek: Phase II D7 studio model, and from those cast the model, which was further embellished. "Greg had a mold of the original, which had been on loan from the Smithsonian during the making of ST: TMP. The "Trials" ship was from that mold and uprezzed.", Doug Drexler later confirmed. Apart from the additional slight hull detailing, reminiscent of the Phase II model, but more subdued than on its K't'inga-class model successor, this model also sported internal lighting, strobe lights, and lights on the crown of the bridge.
Although the producers originally did not want to have the model built, because of budget concerns, effects supervisor for the episode Gary Hutzel pushed the construction through, after he discussed the matter with Jein: "We talked for about fifteen seconds, and then Greg said "Oh, I'll build a shell. You paint it and detail it and we'll put it in the show."", adding "(We) took a little liberty. The original model had no lights on it at all, and it was pretty smooth, with no detail. We added neon for inside, strobe lights and some some lights on the crown of the bridge." (The Magic of Tribbles: The Making of Trials and Tribble-ations, pp. 45-46) Jein's passion for the project however, resulted in a full-blown studio model, produced at a bargain. Jein himself reiterated, "And I said, "You know, in that episode they had not the budget to show the Klingon ship", because a Klingon ship did not show up until the third season, this was a second season show, and we said how about we build a Klingon ship, and he [Hutzel] said, "No, we cannot definitely afford that, we're in trouble as it is!" So I said, "Tell you what, If the day comes when you need to shoot it, give us a call, and if you shoot it, fine, it's yours." So, we actually built the Klingon ship, which was Gary's heartset, so "the hell with it, we're gonna use it!", so he stuck it in the film." (Sense of Scale, disc 2) The build was not entirely without its setbacks, as Gary Kerr, a friend of Jein who was visiting his shop at the time, observed, "(...) Greg's assistant was trying to finish the lighting on the model of the Klingon cruiser – which unfortunately kept burning out." (Sci-fi & fantasy modeller, p. 40)
Neither officially commissioned by the studio, nor intended for further use, Jein's IKS Gr'oth was a one-time use only model and slated to be discarded, but was saved from the dumpster by Hutzel, who took the model into his care. 
The molds Jein owned were put to good use besides the IKS Gr'oth; Apart from this ship he cast from the molds a commercial limited production run of twelve, without internal lighting, that was sold in 1997, accompanied with a certificate of authenticity signed by Greg Jein at the Viacom Entertainment Store in Chicago. In addition to these, two K't'inga-class vessels were constructed from the molds: the K't'inga-class cruiser in TNG: "Unification I" and the IKS B'Moth in DS9: "Soldiers of the Empire".
As the D7-class was only featured in the Original Series era, being replaced by its K't'inga-class descendant, very few other physical representations of the class were utilized on the other spin-off franchise productions. Still, a commercial consumer product showed up to physically represent the class as a display model in Worf's quarters in Star Trek: The Next Generation' second season episode, "Peak Performance".
It was only after the episode was remastered in 2012, that it could be discerned that it concerned a 1989 pre-sale evaluation, or approval nine inch pewter model, formally released the same year, from Franklin Mint's Pewter Star Trek Starships-line. The otherwise unmodified pewter model was later again utilized as a nearly undiscernable display model in drafting room 5 in Mars Station at the Utopia Planitia Fleet Yards in "Booby Trap".
Yet another model of the Gr'oth, this time a CGI version, was used in the 2006 remastered Original Series at CBS Digital, where digital animators worked with the model under the supervision of Niel Wray and David Rossi, for representation of the craft in its respective episodes. The model was built by Finnish fan and digital modeler Petri Blomqvist, and was bought from him by CBS Studios for use in the series. Blomqvist's work was brought to the attention of Wray and Visual Effects Supervisor Michael Okuda by Technical Consultant Gary Kerr, the same Kerr who had visited Greg Jein's model shop a few years earlier. The quality of his work was a compelling reason for the acquisition, as it saved valuable production time. Nevertheless, the digital animators still had their work cut out for them as Blomqvist's model was constructed in the LightWave 3D software, whereas they used the Autodesk Maya CGI software at the time, and had to translate the digital model from one format into the other, which inevitably led to some information loss. Additionally, they had to cut down on the resolution level of Blomqvist's highly detailed high-resolution model, in order to speed up computer rendering time. It, and the other models CBS bought from him for the project, has earned Blomqvist an official "Technical Consultant" credit. (Sci-fi & fantasy modeller, Vol. 26, p. 49-50) In regards to this CGI model, when making its appearances, Okuda noted that "the Klingon ship was basically in two forms. In early episodes, when it was very small on the screen, it was the original version of the ship, which had essentially no surface detail. In "The Enterprise Incident", "Elaan of Troyius", and "Day of the Dove", the ship was reworked somewhat to add surface texture. And, of course, in "The Enterprise Incident" we added the Romulan bird markings."  It should be noted that, in the original run of the series, the design made an actual appearance in these third season episodes of the series only, simply because the physical studio model did not yet exist before that. Yet, for the remastered version of the series, use was made of the opportunity to incorporate the CGI model in those episodes of previous seasons, where the design was implied to be present, and which included the first season episode "Errand of Mercy", and the second season episodes, "Friday's Child", "The Trouble with Tribbles", and "A Private Little War".
Another CGI model was previously-constructed by Doug Drexler for use in James Cawley's 2004 fan series Star Trek: New Voyages (later rechristened to Star Trek: Phase II), in which it made several appearances, debuting in the 2006 episode "To Serve All My Days", but, echoing what happened to its illustrious physical counterpart of the past, first used for the 2005 vignette "No-Win Scenario", that was only first aired in 2011. Nevertheless, that model also showed up in licensed print publications, most notably the Star Trek: Ships of the Line calendars and their book derivative, in particular showing scenes stemming from "To Serve All My Days". Noteworthy, however, is that Drexler chose to have his CGI model mirror the original second "master tooling" model, i.e. the single strip on the warp engines, despite the fact that he had Greg Jein's Phase II studio model in his possession for reference.