botanically plums and prunes are the same thing - and they crossbreed at the drop of a hat - (since they aren't actually crossbreeding) - a seedling from a "prune" may well be a "plum" and vice versa. Generally whether they are classed as prune or plum is usually by shape, texture and whether tart or sweet predominates in the taste.

If I recall the 'Silver Prune' was generally eaten out of hand - didn't dry well - which makes it a plum in many peoples minds.

(Can you tell I studied this stuff at one point?)