Lt Col. Robert J. Icks, Tanks and Armoured Vehicles. Phillip Andrews Pub. 1945
The American M6 was part of an early WW2 program to develop a new heavy tank. In 1942 the first prototype was built but by the end of 1942, the benefits over the existing Sherman medium tank were offset by the drawbacks of dealing with a giant 55 ton vehicle. Forty were built, none left the US and only one remains at the Aberdeen museum.