The M6 37mm Gun Motor Carriage (GMC), was the first variant produced in the ubiquitous Dodge 3/4 ton 4x4 truck family. In late 1941, the prototype vehicles were initially designated T21 37mm GMC and were envisioned to be built in two basic versions: differentiated by front-firing and rear firing guns.  However during testing, it became apparent that the front firing version had many limitations and was dropped in favor of a lone rear-firing gunned variant.  In early 1942, the T21 was standardized as the M4 GMC and was planned to be armed with the more complicated M6 37mm cannon.  Within weeks of the standardization, a decision was made to change the main gun to the more simple M3 cannon.  With that modification, the designation of the vehicle also changed to the M6 37mm GMC we are familiar with. 

Also designated WC-55 by Dodge, this vehicle was very similar to the more common Dodge WC-52 3/4 ton Weapons Carrier, although not identical.  In fact, in 1943 when production ended on the M6 GMC, many remaining vehicles were converted to WC-52 Weapons Carriers.  Despite the WC-55's declared obsolescence, some soldiered on until the end of the war in the more remote war theatres.