The eagle w/ 3 stars over it is the DAS (Department of Defense Acceptance Stamp - post WW2).
The large P is a "proof marking." Meaning that the rifle with this stock had a "proof round" fired through it and it passed the government's test.
But............as you know..........parts and stocks get switched around frequently. Do you remember reading this on the CMP site?
Over the past 65 years, most M1 rifles have been arsenal rebuilt, refinished, rebarreled or repaired at least once and often several times. Most will show signs of service (often considerable) and replacement of various parts. They are seldom encountered with all original parts and original finish as delivered from the manufacturer. Such "original" rifles, even in well-used condition, are highly prized by collectors.
IF you're really interested in your rifle and parts............invest in some good books..............
http://www.scott-duff.com/DuffBooks.htm#m1Garand
There are two books on the M1 by Duff.
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Here is a free web site with more info on cartouches..............
http://www.garanddata.com/owbase/ow.asp?Stock+Cartouches
Aloha, Mark