There should be hooked prongs on the back/bottom of the bracket, that reach under and grab the bumper from the bottom holding the bottom of the bracket in place, hence there are only two holes in the top of the bracket which you drill into the bumper to hold the top of the bracket.
There should be notches in the bottom of the bumper which these hooked prongs fit into, feel for them, these notches center the bracket left/right on the bumper. You don't need to drill large holes in the bumper for those two screws, a small diameter drill bit I think I used 3/32 inch is fine, the bumper is only thin plastic in this area so don't drill the holes too large or over tighten.
The two brass threaded inserts, who knows possibly used for plates in other countries. In the USA you will use the 4 plastic screw holes for the plate.
