A sort of sliding stop that sets the position of the buckle when the belt isn't being worn. On my 'new' car, one is present - luckily on the driver side - but the passenger one missing. Which means a passenger has to scrabble about on the floor to find the buckle.
The one that is there looks like it just clips over the belt and stays in place through friction. I've looked on Ebay but don't know the exact name so got tired looking. I'd guess most belts are the same width so a generic type would be OK?