From TAS,
The Slaver Weapon:
SPOCK: Stasis boxes and their contents are the only remnant of a species which ruled most of this galaxy a billion years ago. Their effect on science has been incalculable. In one was found a flying belt which was the key to the artificial gravity field used by starships.
It's a shame that our government withheld this discover in our reality. In the Star Trek alter-reality, they exploited the discovery to build atomic powered, gravity field drives in interstellar spaceships complete with artificial gravity in the 1990's. In
Space Seed, the Botany Bay was atomic powered with artificial gravity launched ~1996. Since it was found many lightyears (hundreds?) from Earth, it must have had a FTL drive or at least a high-STL drive, perhaps an early gravity drive which propelled the craft until its atomic fuel was exhausted.