Were these spots usually on the tops, or were they all over? Sun/heat would be worse on the upper sides.
Also, did the fruits on the bottom start out looking like the Shiro in your earlier post? In other words, did the skin first get the darkened look and then shrivel? The damage in the lower picture looks like it could be a bug of some sort, but if it started out as a bruise thing I doubt it.
My best guess at this point is you have some form of plum scab in those Ozark plums. Next best possibilities are heat damage and bug damage.