If legitimate, the "M1903A2" indicates the barreled receiver was used as a sub-caliber device for 37mm, 57mm and 75mm artillery pieces.
The "asterisk" in the muzzle crown does not look like a legitimate Star Gauge marking to me. Look about 1/3 of the way from the receiver end of the top of the barrel. Is there a letter and 3-5 numbers stamped there?