I used to think it was a trait of Match rifles as well, but the more I investigated it, it's present on all late Springfield Receivers, match or not. LT1 is correct on that. I think now it was just a hardness test on the receiver, done by Springfield.

The weird thing is, I've seen many of them on Low Number and RIA Marine Rebuild Rifles as well. Some have been very early serials. So I also think the Marines or Navy might have done it for a short period of time somewhere probably around WWII when all those changes were happening.