Pro Libertate wrote: I'm sure someone who is more knowledgeable than I will be along shortly, but it was my understanding that the circled "P" proofmarks on the back of the stock were done away with later in production (probably as a time-saving measure). It's very difficult (some might even argue downright impossible) to determine with absolute certainty whether any given barreled action is original to the M1A1 stock its paired with. Without provenance it's hard to definitively say one way or the other, and this is unfortunately the case with many an M1A1.

I wonder if the proof stamp was eliminated because some stocks were getting damaged beyond acceptance standards when they applied it?

Well, did not use a magnifying glass, but placed another pair of glasses on top of my regular ones to kind of make one! Did not need even one pair of glasses when I was 20! What happened? I digress, no mark of any kind. Do not think sanded or worn, just never applied. I guess maybe a replacement part at some point early in life, or just missed getting stamped.

Might have been sanded off. Might not have been marked on stock

  But again, If damaged in shipment or training. that's why batches of Carbines all had replacement parts

  Shipped with them.

Lots of good replies in this post. Let me try to stitch it together.

M1 carbine stocks in general the proof P was an early requirement and seen on Winchester, Underwood, and IP/S'G' stocks.

the P proof on the stock was no longer required at some point in 1943. It would seem that someone at Inland did not get the memo as we see the P on the bottom of the grips of M1 stocks till late 1943.

our webpage on the M1a1, which I linked above states:

(Note, When looking for information on a web page it is useful to use the search function to find a word. On your keyboard hit Ctrl and F together and a search box will open)

CCNL 361-5 on the 4-5 million (third) block states the P proof  appears to have been discontinued at the end of the second serial number block.

It also mentions the Greek returns appearing to be in original condition and no P on the rear of the stocks that were typically seen on the first production run of M1A1s. Note that most of these M1a1s from Greece were in the 4-5 million range.

CCNL 348-14, 15 has an article on the Greek returns and also states most M1a1s did not have the P on the back of the stock or grip, and none of the Richardson made grips have the CC on the side of the grip.

I have one of these Greek returns in the 8k range (SG subcontract) with a 8-43 barrel date. There is no P on the back of the stock which had me scratching my head back in the day. This seems to fit the web page as a second run of the M1a1s

Your carbine with a 12-42 barrel date would seem to be a first run. So the question is if someone dropped the action in the stock, or if an original. You may want to post some pictures and a data sheet for a better idea of what is going on. Pictures of the rear tang and the recoil plate may be very helpful.