Ooh, here's what's credited as the tropemaker: Jean Harlow in Hell's Angels. I just spent way too long trying to work out if the actress is wearing a girdle; my guess would be no.

Have you ever seen Gosford Park? There's a scene where it's really clear that a character basically uses a coded version of "let me go put in my diaphragm" as misdirection. I find the shift of contraceptive use in the 1930s really interesting. I don't remember if I posted about reading Marie Stopes' letters? Anyway I did and it was really interesting.