Proof Testing and Proof Marking
Before leaving the manufacturer, M1903's and M1903A3's were proof tested for safety. Proof testing was done by firing several cartridges loaded to produce a pressure substantially greater than that produced by the standard issue military cartridges that would be used in the rifle. If the rifle survived this test of its strength the letter P enclosed in a circle was stamped on the stock just behind the triggerguard, and the rifle was ready for acceptance by the military.
The earliest proof marks used on Model 1903's were a script letter P which was later changed to a block letter P.
U.S. Model 1903's and 1903A3's will sometimes be found with two letter
P's stamped behind the triggerguard. The second letter P is believed to have been
used when a rifle when through a complete rebuild. Because of the extent of the work
it was likely proofed a second time. If a stock is stamped with two letter P's there
is a good possibility that it will also have other marks indicating an arsenal
rebuild. (See the section on arsenal markings)