Only some varieties of plum will turn into a prune when you dry them. Many will become a moldy pile of fruit flies instead.

First of all, 'prune' is French for 'plum' ('prugna' in Italian). Plums which are suitable for drying are named 'prunes' and even 'prune plums' in English. My tree is an Italian Prune (Prunus domestica) as you half mentioned. Notice that the Latin isn't "Plumus" and is "Prunus". I grew up with Purple Leaf plum trees (Prunus cerasifera). They would rot. I haven't seen fermented Italian Prunes in my yard, even the ones that the squirrels and crows have taken bites out of. You may have figured out by now that only in modern English is the fruit name conflated with the specific dried fruit product. This conflation is the point of my original comment on a narrow interpretation of the word 'seam'.