72 seat belt question

[T2R9]

I have a clip on both front seat belts that mount under the sill plate. The plastic seat belt housing slides into it and the clip holds it in place low to the floor. These are on my 72 Demon which is a pretty original car.

Is this a 72 only part? Never seen them before and only owned one other 72 A-body and that was 1985!