The 747 was a large four engine commercial passenger airplane that was in operation on Earth in the late 20th century.
The airline British Airways (BA) operated 747s. On a large billboard in San Francisco, visible from Columbus Avenue, BA advertised its services with a picture of this type in flight.
In 1986, the humpback whales George and Gracie were transported from the Cetacean Institute in Sausalito, California to the Bering Sea off Alaska via a 747. The aircraft was specially modified to carry the two humpback whales that distance. (Star Trek IV: The Voyage Home)
It is likely this was a reference to the Boeing 747, one of the largest commercial jetliners in the world.