Seat belt clips

I have an early 68 Dart with bucket seats and it doesn`t have the seat belt clips on the front or back seats. The front seats had been recovered so I don`t know if they originally had the clips or not. The back seats showed no signs of ever having them. I`d like to have them but don`t want to put them on if they weren`t offered yet. The car was built 8-28-67. When did they start installing them?