There was a backup light option in '55 (I have a set on mine that were NOS), but they were simply mounted to the fender with a rubber base that was angled to fit the contour. I have seen the fenders you describe with the mounting pad stampled into the fender. They are for a later truck, most likely the '66 the John mentions. Here's a
pic of the '55-'59 style, which uses the same lens, bezel, and base as the '50-52 Chevrolet and Pontiac Passenger car backup light. The rubber mounting block is Truck specific.
