Yep, you got my hint about the "McGuffin,", that being the maltese falcon, the object of the quest in that film. Another alias she had in that film was Mrs. Wonderleigh. There was a third, but I can't remember it at the moment.
Sinclair, of course I knew it was Mary Astor, but thought I would give someone else a chance, and a hint.
Hypatia, that photo of Mary A. does look a bit like a very young Margaret Dumont to me. Also looks a bit like Lillian Gish. That photo doesn't look much like Mary A. did in The Maltese Falcon!