Well, don't worry too much.
Mine's R9XX, and the crack went all the way from front to back of the grasping grooves and around the rear on the left side of the slide (not a through-and-through crack, that is, not all the way through the steel, but a looooong, if not too deep, crack).
Anyway, maybe it means QC is worse with LATER than with EARLIER serial numbers?
Or maybe it's a fluke.
In any event, it suggests to me that either the gun is running near the limit of its material stress tolerances, or that there's a QC problem. I suspect the former is the case, but neither choice is very good, of course.