My bumper is too high
The rear bumper on my 1953 Chevy pickup sits so high it covers the bottom half of the license plate. The brackets look to be original. Do you or your readers know whether there were multiple versions?
I wonder if someone installed the bumper on your truck at a later time and took it from a different year or model. As I recall, the standard bumper irons from that era sloped down slightly to allow for the license plate. As you know, rear bumpers were extra cost options on pickups from that period. And there was more than one choice. You could have a painted bumper or pay a bit more for chrome, and you could have a heavy-duty welded steel bumper with a non-skid boilerplate on top so you could stand on it while shoveling stuff in or out of the bed. If you can find a set of the sloped bumper irons or have a set made, that would solve your problem.