A few years later, I finally checked these files. The daily production report and the call sheet both indicate a "Libby Erwin" did one day of work on Monday, November 11, 1968. She was on set from 9:30am until 6:45pm. (Incidentally, Memory Alpha says this work was done on stage 10, but it was in fact stage 9).
Unfortunately, the cast sheet is missing for this episode, so there's no address or other identifying information that I could use to track down Ms. Erwin (if that was, in fact, her real name). ProQuest and Newspapers.com both show that a "Libby Erwin" was active in Los Angeles real estate in the 1970s and 1980s. Unless the cast sheet for this episode shows up, not much more I can say on this topic.
This woman, despite the resemblance, was probably not Erwin Armen — but I can't say that definitively.