They may be drive screws…. Common for them to be used for the plates. Most times there is a thru hole so one can pop the drive screw out from the inside.  I have seen them without the thru hole as well, but that seemed less common.

If you can get a good pair of vise grips on the head you “might” be able to turn the drive screw out…..if memory serves it should unscrew counter clockwise.  Just some break-time thoughts.

On that style of ID plate, I don’t believe the rivet holes go all the way through the bracket so you can’t get at them from the underside.  They are in fact drive screws (like a rivet with coarse threads).   You might be able to carefully get a thin blade screwdriver up under the head of the screw and pry it up just slightly, then grab onto the head with pliers or vise grips and remove it by turning it counter clockwise.   If not, you would have to drill them out – hard to do on such a small diameter fastener.