Rear Bumper loose?

on my 66 dart, the rear bumper is in real nice shape. Except it is sorta loose, meaning I can wiggle it up and down? Is that right? I tightened all the nuts, but still wobbles. You can see on the outer edge there is a hole for a carriage screw, but there is no screw in it. Nor do I have any access from inside to that hole, so no idea what is would do? Or maybe this is not the correct bumper? Thoughts?rear - 1.jpg