I think the story goes something like this. Way back in the 1960's when the original series was in it's first year, the producers hired a model maker to build a ship for a new race that was being introduced, called The Romulans. The ship was the original "Bird of Prey". It was called that, because there was a huge painting of a bird of prey on the underside hull of the ship. Now sometime after the episode that featured this ship was aired ("Balance of Terror"), it was found that the model maker who had built the Bird of Prey was not a union worker. There was some type of problem with the unions, so his models had been returned to him, and could not be used in future episodes. There have been rumors that this man smashed his work out of anger. Now comes the 3rd season of the original show, where the producers needed a replacement for the romulan model. To save money, they just used the Klingon D-7 model, and put a bit of back story in the episode that the Romulans are now using Klingon designs. FASA games went further in it's game books saying the Romulans and Klingons made a trade. D-7 type heavy ships for a trade of Cloaking technologies. When Star Trek III was in pre-production, the original villains where going to be The Romulans, not Klingons. The model makers had already made the new "Bird of Prey" (that is seen in Movie 3,4,5,6,7, and countless episodes of the T.V. shows from Next Generation on). This made for more confusion. Now Klingons had Romulan ships. I lost track after that.