I have a theory that I think holds water.
The blonde is an uncredited extra who never played a significant part on Mannix. Having been a student intern on Quinn Martin TV shows, I know that series production companies pretty much used stock companies of extras (bit players / background players / atmosphere, etc) who were around the set frequently or all the time; often, "wild" footage would be picked up for unknown potential use, just in case. I came across a Mannix fan site where the name THORDIS BRANDT came up on the same subject of the title dancer, but not followed through on. [monsterkidclass.horrorforum.yuku.com], and that's who I think it is. She appeared in an uncredited part in Mannix, season 2 ("Pressure Point") as Miss Hampstead.
From Wikipedia: A bit player for most of her career, she appeared in such TV shows as Burke's Law "The Girl from UNCLE", "The Green Hornet", "Mannix" and a brief appearance in the 1967 episode of "The Fugitive", The One That Got Away. Thordis' appearances rarely required her to speak.[2]