The pilot vessel used by Balok was a small ship capable of enormous power. It controlled the flagship Fesarius and was equipped with a tractor beam. Other than Balok himself, no crew was necessary.

On stardate 1514.1, the USS Enterprise was forcefully towed by the vessel on a heading to Carpi and could only free itself under great risks. (TOS: "The Corbomite Maneuver")

The script of "The Corbomite Maneuver" made it clear that Balok's pilot vessel was a section of the Fesarius which detached from that mother ship.
The set for the interior of this craft was built on Desilu Stage 9. (Information from "The Corbomite Maneuver" call sheets) Clint Howard – who, in his childhood, played Balok – approved of the set. He remarked, "A great thing about the set is it was built to my size... and everybody else had to hunch down, which I thought was pretty cool." ("Inside the Roddenberry Vault, Part I", Star Trek: The Original Series - The Roddenberry Vault special features) Several walls from this small set were subsequently reused, to represent the M-11 Starbase Club in "Court Martial". (The Star Trek Compendium, 4th ed., p. 46)
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