Apparently there are several cultivated varieties of plums which do ripen in
early summer. My wife suggested that Jim Kamas, Ag. Extension Agent for
Gillespie County, would know which plum varieties are best for jelly. She
also provided this information:

"Here's a list of peach farms in the Fredricksburg area. Quite a few also
list plums on their websites; however, looks like most of them are harvested
in June. They don't give the varieties of plums grown.
http://www.texaspeaches.com/ "
jjhnsn