The original model for the Romulan Bird-of-Prey from "Balance of Terror" was no longer available, but TOS season 3, episode 13, "Elaan of Troyius", had already been produced, and using the Klingon ...
In TOS season 3, episode 13, "Elaan of Troyius", it's not Kirk that the Klingons are after, despite it seeming that way, but ...