I don't know if the punch mark was used when that rifle was made. All the info I have seen on that mark dates from after the heat treatment change, so they might have begun using it at that time.
In any case, that rifle was proved after manufacture and survived. But the proof load, like the service load, uses powder that gives a "push" and the receivers have no problem with that. But it is like a beer bottle - you can lay it on the floor and stand on it or put internal pressure on it without it breaking; but tap it with a hammer and it will break.
One receiver failure I know of was with a load of 9 grains of Bullseye and a round lead ball. The owner was using it to shoot rats. He had fired that rifle many times with standard loads with no problems, but the sharp rap of the fast-burning Bullseye was too much.
In my own experience, I held the receiver in my hand and rapped the left side smartly with a hammer; the receiver broke into three pieces. Yet, that receiver was from a rifle I had fired the day before with standard GI loads.