[Mr70]

Those are seatbelt buckle retainers and generic for ALL Cars.
They were not a factory RPO,but a Dealer Accessory only.
They were screwed into the seat spring frame lip mounted with sheet metal screws at the bottom base of a Bucket or Bench seat,even in the back seat too.
I've come across many of these in cars in my past,and they are an absolute B**** to take off.You just about have to remove the seats and turn them over,to get at the screws.
Imagine sitting in a back seat,when mounted,these would touch your heel/back calf area & were quite an annoyance actually.
It's much easier to tuck the unused belt into the seat crevice.
They came out in the 1970/71 time zone & were used on Monte Carlos & Caprices mostly.Somewhat of a Luxury convience item was their claim.
They discontinued these in I believe 1979?