Seat belt holes?! Please help never seen this before

I have a 1971 demon i just bought that has to have interior put back in and all the seat belt hole locations have perfect factory circles with no threads. The holes are the size of a quarter. I have never owned a demon mostly B-body cars, is there a bracket with thread hole that is meant to be bolt screwed in over the quarter size hole as a guide for where the bolt recesses into? Im stumped? Need some guidance here