<< I wonder if they got Barbara Rush's "doppelganger's" little black dress from Putnam's closet? <<

LOL and I've wondered that myself -- whose closet yielded the black cocktail dress, Putnam's or one of the two linesmen? Because, yes, the movie does (subtly) indicate that when the Xenomorphs take human form, they must be initially naked -- all the talk of stolen clothes. And the script makes a point of stressing that at one point, George (Russell Johnson) should be seen in background, taking off his coveralls to reveal street clothes, so that a few minutes later, when we see both the real George and Glob George, they can both be dressed.