1969 A Body (Barracuda) license bracket

The above pic shows it all. A bolt head with a flat washer, then the bracket. Under the bracket is the bumper ear with bolt through ( not actually shown ), and what is very likely a kept washer type nut.
Mine didn't include the OEM hardware but I'm not on assembly line. I can use sperate flat washer, lock washer and nut or nylon locking nut.
The large square openings that allow this view,,, for all practical purposes are wrench openings and/or venting. Reduces wind loading, waterfall effect from beneath the plate, whatever condition might have occurred.
Does look as though the rubber bumper is missing from behind that plate. Plate laying against bracket there can create a strange little vibrating/buzz noise.
My aftermarket frame is much like a picture frame, it has full back. I put a bolt through it where the rubber bumper was removed. Lower hardware also through that back plate. The plate is simply captured by the frame like a picture in a frame.