Willys MA and MB (standard & composite) and Ford GPW composite bodies were made by American Central Manufacturing (ACM). Ford had a own production for their early bodies. Willys did use the ACM bodies for their early and late bodies. The early ACM bodies can be identified by the round depression for the tool locker latch, while the Ford GPW had a rectangular depression. 

Body numbers are stamped on the left body gusset. The early body had a triangular toe board gusset with the number stamped on the left side, the gussets on the composite bodies had rounded corners with the number stamped at the front . 

Byuing m forla you are able to calculate an approximate serialnumber. Impont is ofcource that the body of your jeep is the original boby