to me it's a M1903 Mk1 stock that was put on that rifle either during a refurb, or when the gun was made a shotgun. The barrel would have been removed to smoothbore it, so no telling if it's the original barrel, but it's not inconceivable that an early 1942 RA would get issued, get damaged, go back for overhaul in late 42 or early 43 and get a late 1942 SA replacement barrel installed.