The Romulan Bird of Prey was designed by Wah Ming Chang in just two weeks’ time.
He also built a small model of the ship, although it isn’t clear if this was the internally-lit studio model that was sent to Film Effects of Hollywood, where the footage was shot for “Balance of Terror” and “The Deadly Years.”
Wah’s contributions to Star Trek were largely unknown at the time. He worked for the show independently from his home studio in Altadena, California. Due to union restrictions, he wasn’t credited on screen, even though Wah supplied most of props and quite a few of the costumes on The Original Series, including the Romulan helmets in “Balance of Terror”.
Producer Robert Justman, who considered Wah’s work superior to anything members of the propmakers’ union could come up with, invented a ruse to make it appear that Wah’s creations were bought for the show off the shelf. Wah’s true involvement in Star Trek only became known a decade after the show aired.
After filming, the Bird of Prey model disappeared, which may account for the Romulans showing up in Klingon D7 cruisers in the third season episode “The Enterprise Incident.”
It may also have been the case that the producers wanted to display the D7 model as much as possible as a courtesy to the model kit company Aluminum Model Toys, which actually paid for it.
In any event, the model’s whereabouts are unknown.
The inconsistency of Romulans piloting Klingon ships was partly mended in the remastered version of “The Enterprise Incident,” when an original Bird of Prey was added to the scene where three Romulan vessels surround the Enterprise. Romulan markings were also added to the underside of the D7s.
The remastered version of “Balance of Terror” includes a new shot showing the aft three quarters of the Bird of Prey.
Sources for this story include: Herbert F. and Yvonne Fern Solow, Star Trek Sketchbook (1997) and “Romulan Bird-of-Prey (23rd century),” Memory Alpha