[cericd]

These brackets were originally used as rear license plate brackets on trucks with no rear bumper. Back then these trucks were not required to have a rear bumper from the factory. Step bumpers were added by dealerships (to promote their dealership), sold by aftermarket companies, and offered as a factory option (this option would be listed on the SPID). Seems like the factory option bumper is really rare.

Here's a pic of what the unmodified mount looks like mounted in the proper location for a truck with no rear bumper. However, someone mounted a bumper in from of the mount without removing it.