First thing to look at is the wood. All genuine M1A1 stocks are made of walnut. All GI M1A1 stocks have an OI stamped in the barrel channel just forward of the magazine slot. Early models as well are marked with a circle p on the rear of the stock portion. Not all however, as one in my collection is marked with a small circle p on the left side of the pistol grip.
Later models have a large crossed cannons on the right of the pistol grip.
Any pistol grip not marked with an ordnance belted cannons can be considered a replacement grip. The upper wood of the stock was never marked with manufacturers marks or the ordnance mark on the right side like M1 stocks. These are not only fakes but bad fakes. Winchester never made M1A1's and the markings are in the wrong place. Original M1A1 stocks did not have a selector switch cut out for M2 usage.