I owned a 70 that had those clips.
It was an Aug 69 build, maybe that plays a role that it was a 69 holdover thing?
GT No 3#
View attachment 533655
FWIW, those clips are the narrower version for the "deluxe" seat belts. These guys sell repops for the rear seat retainers.
All Mopar Seat Belts - Bill and Robin Edwards - Upholstery Mods and More