What is it? Is this something you screw onto the outside of the fender to mount another item/reflector over top? Is this for the turn signal in the grill?

This, and these reflector pads are used to mount the reflectors to the Bronco. The pad sits under the reflector and is held in place by the reflector and its screws. The pad and reflector is specific to the location on the Bronco. The grill pads are #9353.