Answer: Based upon your growing region and the description of the fruit, I'd guess it's one of the Gage plums. Maybe Bryanston Gage. There's a book with pictures and fruit descriptions that you might find helpful, called Taylor's Guide to Fruits and Berries, published by Houghton Mifflin Company, 215 Park Ave. S., New York, NY 10003. ISBN# 0-395-71086-3. Hope you find the true identity of the plum!
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