Gee Dave...I really wish I could be of more help.This orchard tending stuff is all new to me..just this past year!
Did want to write you about our plums.I don't know the name.
Growing on thier own root stock,med sized,extremely hardy! And get this...the fruit is pale green!! Sweet,firm yet tender.Makes wonderful jam/jelly.
Maybe someone knows the name? Want me to save you seeds this summer?
Maybe your green plum is what we over here call a Greengage? My grandfather used to have one. And the best plum over here is often considered the Victoria variety. We used to have one - huge fruit, the size of a large egg, reddish-orange skin, juicy orange flesh, on a small tree. We had one in our last garden, but it fruited itself to death. Here, I've planted a Damson - small black fruit, quite tart, good for pies and jams. Suppose what does well for you will depend on your local weather/soil/climate.