Here in Mass, if you have on older "green" colored plate you only need the one (which I have thankfully, the front looks so much better without).
As for the rear plate mount, I too saw that it had only the bumpers. But I also noticed that the middle of the bumpers were hollow. I found a couple of small sheetmetal screws and was able to thread into the center of these bumpers (I have caps that go over the screws for the plate frame, snaps over a plastic cup that is under the screw).
You'd never know it and it's pretty secure.