I believe the "US" on the butt indicates that stock was on a rifle inspected for DCM sales, and graded "unserviceable sales". Most of the unserviceable rifles had defects of a minor nature, or had not been completely inspected. Serviceable rifles were marked "SS", "serviceable sales". Later they did away with those markings and ALL M1903A3s were sold as "unserviceable / safe to fire live ammunition". I have a similar marked stock on a M1903A3 that I know is not original to my rifle.