US M-1 57mm AT gun on M-2 carriage The idea of manufacturing of the British 6-pounder in the US was expressed by the US Army Ordnance in February 1941. The US version, classified as substitute standard as 57 mm Gun M1, was based on the 6-pounder Mark II. Production started early in 1942 and continued until 1945. The M1A1 variant used US combat tyres and wheels. The M1A2 introduced the British practice of free traverse, meaning that the gun could be traversed by the crew pushing and pulling on the breech, instead of solely geared traverse, from September 1942.