I have also seen the letters "MBC" stamped on the back of numerous bumpers that were attached to "original" trucks. I've seen it on trucks in boneyards, and also on older "restored" trucks. My guess is that it was maybe a replacement bumper that was installed "back in the day" by the dealership or whomever. I can say that those bumpers with the "MBC" stamping fit alot better than any new reproduction bumper that is on the market today.