Looks like it could be an early modern round brilliant but might be described as transitional by some. I do believe that there are many early round brilliant cuts out there that remain in their original settings. I think we often expect to see earlier style cuts in antique/vintage settings but I suspect that during the 1920s and 1930s there was a considerable range of cut styles (including early modern brilliants) available. I have a ring with its original diamond that has never been removed from its setting (see my avatar) that was made in the early to mid 1920s. While the diamond does appear more modern than vintage there are a couple of features that I guess one might associate with vintage stones: bruted girdle, medium open culet, higher crown, & smallish crown facets.