From the book mentioned above: Proof Firing Cartouche:The "P" proof indicating test firing was stamped on the forward portion of the pistol grip. Five variations of the firing proof mark were used, see figure..........
H&R stocks showed the circled "P" with serifs to circa serial #4,800,00 and the sans serif circled "P" thereafter (Figure B)"
IHC used the circled "P" without serifs (Figure B) to circa serial #4,660,000 in conjunction with the Ordinance Department's crossed cannon acceptance stamp for their first few thousand M1s....
........Note serifs or lack of serifs on the "P" mark. The M1 Garand rebuilding program began at Springfield Armory in the fall of 1945 and was officially known as the "Clean and Repair" program.