I think those might actually be frosted glass cups.

Here is the best copy I could find, and the flared edges do not look like plastic cups - also the shiny inside and matte outside looks more like glass frosted on the outside.

PP was not in commercial use until 1957 (not sure when it was first used for cups), PET disposable cups weren't a thing until the 70s. Disposable cups in the 50s would have been wax paper.