The casting number 9605815 identifies it as a Retainer, Front Seat Shoulder Strap, left hand side. There are several of these up for sale on eBay etc. for ‘72 GM A-body cars. It has a twin 9605814 for the right side.
Was not able to find where it was mounted. Looks like it attached with 3 sheet metal screws. Maybe it goes on the roof rail to stow the belt when not in use?