The early manufacture Auto-Ordnance, Bridgeport guns, like their Savage counterparts, were fitted with the finned barrel and the adjustable Lyman rear sight. A similar AO prefix serial number was applied to the rear of the trigger frame. The letters AO preceding the weapon’s serial number on the left side of the M1928A1 model’s receiver will easily identify an M1928A1 Thompson that was produced in the Auto-Ordnance’s Bridgeport, Connecticut plant. The 1928A1 Thompson Submachine Guns being produced at Auto-Ordnance’s Bridgeport plant were identical mechanically and in appearance to those being made by Savage in Utica, New York. As a result, the Auto-Ordnance Corporation opened their own factory in 1941 to help fill the Ordnance Department’s requirements for the weapon.

Due to the complex construction of the Thompson, Savage had a difficult time keeping up with the ever-increasing demand. In 1939, Savage Arms was contracted by Auto-Ordnance to manufacture the weapon.

During WWII there was an overwhelming demand for the Thompson Submachine Gun, to supply both U.S.

Para ordnance serial number date