How is it titled? If it is titled as a General Motors Truck, then it is possible somebody converted it to a Chevrolet in appearance (or totally) and overlooked the dash trim and heater. The ID plate on the door jamb will also probably show General Motors Truck if this was done. Or maybe the upper Chevy trim piece was damaged and somebody replaced it with what they could find in a junkyard and got a working heater in the deal. In the mid 60s and up until the early 80s it wasn't uncommon to see people convert GMC trucks to Chevy trucks around here.
