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Thsi gun has the rare hand stamped A1 Suffix behind the roll marked M1 Prefix indicating the receiver was actually intended to be used on an M1 weapon...see below
From: Small Arms Review, Frank Lannamico
"On many M1A1 Thompson guns manufactured at the Auto-Ordnance, Bridgeport plant the A1 designation appears to have been added by hand stamping. This has led many collectors to believe that these were originally M1 weapons, which is true in part. The Auto-Ordnance, Bridgeport facility had a difficult time when changing over from M1928A1 to M1 production, resulting in a several month delay in the start of their M1 production run. This delay was followed by another, when the weapons were rejected by the Springfield Ordnance District inspectors, because their full-auto cyclic rate exceeded the specifications. By the time the problems were resolved, and the Bridgeport plant M1 guns were finally being accepted by Ordnance, the M1 had been superseded as Standard by the M1A1 model. Most all of the receivers on hand at Bridgeport had already been roll marked M1, but were upgraded to the new M1A1 configuration by replacing the bolts. This necessitated the hand stamped addition of the A1 suffix on these weapons at the factory."