The assembly line procedure is to attach both brackets to the rear bumper, with bolts, washers (they're called spacers by Ford) and nuts, then that entire assembly to the car. The clearance to adjust and tighten the nuts between the bumper and body is tight, almost impossible to tighten without scratches when the bracket hardware is tight. I believe that Ford had a fixture that held the brackets and bumper in position to allow the hardware to be installed and adjusted to match the body. It's also possible that the license plate light assembly was installed at that time too.
Jim