Rattle

I have a 67 fastback and I finally found the source of a very annoying rattle. It is the passenger side seat belt anchor for the rear seat. My question is I cannot find the under side nut to tighten this anchor. Is there some sort of secret hole up in the undercarriage that I cannot find.

Please help........

Well I looked under there again and there is no bolt to speak of. I had once heard that the bolt actually goes into a plate and you have to remove the plate to get to it. Is that true? If so some pictures would be real helpfull.