Since the locking block is a MIM part, I am pretty sure that that is where the metal is injected into the mold. I think it is where the sprue attached to the part. If you want to see if the gun has been fired a lot, look at the foam inside the trigger return spring. A new gun will have a pretty white foam piece, 100 rounds later it will be grey, and thousands later it will be black...

ETA I just looked at a new in bag block that has never been installed, and it has the same mark. The sear block also has one, but you can't see it without removing it from the frame.