The proof mark on U.S. rifles of that era was a "P" on the bottom of the pistol grip. On yours, the circled P is the original, the other is the second proof. The reason was that American receivers were so hard that it was not practical to use a proof stamp on the receiver (as the Germans did); they would have needed a new stamp about every ten guns!
The drawback of that system was that the proof was on a replaceable part (the stock), but since the Army didn't consider the proof important once the rifle left the factory, that didn't matter. It was just another "check off" before the rifle was ready for shipping.