the screws should be going through the inside center metal strip into the outside center strip which is threaded.
Perhaps you could try one of the "clips" from a vendor that is tightened to the center strip and then attach the mirror to it??
...and on some models there is a hole in the inside center strip that a mirror arm can go through and be tightened up.
I'm sure some more bolters that are more familiar with your problem will chime in with their solutions.
