It means that the stock was put on the rifle by someone other than the manufacturer or the US military.
The serial number dates the receiver to February 1945. Everything else.....we don't know.
EDIT: Without photos, all we can do is guess what you have. We don't even know if your M1 rifle is USGI.
In almost all cases, a USGI stock will have a proof P, as well as a cartouche and/or arsenal rebuild stamp. Usable stocks were often reused for arsenal rebuilds, & they were often sanded. Multiple P's & markings can be found.
We can sometimes identify USGI stocks based on the shape & certain markings. All IHC stocks have a Julian date stamped in the barrel channel.
I think the Navy marked there stocks with an A when they did a rebuild on the Garand. I tried to do a search on CMP Forum where I read it and there was to much data to go through. If you are looking for a GI stock keep looking for more information that may b what you have.