On my 50(something) CHEVROLET 1/2 ton panel, the data plate which is mounted to the door post, says: General Motors Corp. Pontiac Mich. The plate is black, the serial no. starts with a KP prefix.
I think that Chevrolets had blue data plates. Is it possible that somewhere along the way this truck got a GMC data plate riveted on? If so, is there any other spot on the truck that may have a serial number stamped?