And didn't they use D-7s in "Enterprise Incident" b/c they couldn't find the original "bird" model? Read that somewhere.
Nerys Myk wrote:
I heard it was "lost" on purpose.
The real reason had partly to do with the first quote and nothing to do with the second. But mostly it had to do with economics.
The Romulan vessel model was only ever used in "Balance of Terror." Any subsequent episodes where it was featured was just stock footage from BoT.
For whatever reason, they did not have possession of the model by the beginning of the third season, but they had just budgeted for the building of the Klingon Battlecruiser. However, the first episode of the third season televised was "The Enterprise Incident," and they needed three Romulan ships for that. There wasn't enough in the budget to build yet another model right after the new battlecruiser, so they basically used the new Klingon ship and added a vague line to the script saying that the Romulan were using "Klingon designs." It's as simple as that.