Has anybody seen similar markings on a stock like these?

[RCS]

Here is photos of an early SA SPG stock converted to the trapdoor butt plate configuration, note that also made two equal sized holes. The original stock has the large over small hole and they took the time to convert it. The P in a square proof is late into post war period too

[Ms15710]

Don't mean to revive a rather old thread, but I was going through my collection of GCA journals when I found this memorandum from the 505th Parachute Infantry, dated April 13, 1944


While obviously an I and not a U, the stampings look identical and are in the same exact spot on my rifle as in the diagram. Could this possibly be what the rifle stamping represents?