I don't want to rain on your identification hopes, but I gave up on plum IDs a few years ago, after consulting the best
local book on tree identification. It basically told me that there are five types of wild plums in Oklahoma and that all of them hybridize freely with each other and with domestic plums. In the gentlest possible terms it warned that plum ID was virtually impossible without the resources of a botany lab and quite difficult even with one.
In your shoes if I really wanted to know bad
enough, I think I'd contact my county extension office and see if they know of a local plum expert.