Is it that square looking hook? My car has the square-ish hook still there. I always thought those were the hooks you still see in modern day cars for hanging up your suit jacket and dry cleaning.Thanks for clarifying your car. If you only have lap belts, then someone has removed the shoulder belts (for the front seats), which attaches to the lap belt at the slot in the tongue/tang. There is a mount up by the quarter window. You can see this if you scroll back through this thread as it is on my 71 Mach 1.
Lots of pictures posted. Look at P2, April 9 for close up of what I think you're talking about.