Typically in a high echlon repair/overhaul/rebuild the facility applys it mark to the left side of the buttstock. In some cases its on the right closer to the wrist.
Two things:
1. Inspect the stock for traces of stamps using a strrong light source held obliquely to the surface.

2. The A3 has metal ring that surrounds the barrel at the receiver end of the barrel guard. The stock has to be relieved to provide clearance for this ring. You may wish to examine the notch and try to determine if it was machine or hand made. if it looks crude it may indicate a conversion of an 03 stock.

Regards,

Jim