Do the taste test,Blisters. Mirabelles are so much sweeter than other plums.
Picked similar (not with a pointy bottom though-((plums not me)) ) and have made jam, fruit liqueurs,leather and frozen some.Loads more to pick, I am surprised more folk don't pick for free.
If you are apprehensive about tasting,give to someone else to taste-then watch them closely......... ;D