Trying to understand these front seat belts.

The patterns of the custom covers resembles 1970 Charger seats & they were available with the buddy seat - http://www.hamtramck-historical.com/images/dealerships/colorAndTrim/1970/70_Charger0005.jpg

If you look under each bottom in the springs & inside the removable backs, you may find a broadcast sheet or two for the car they were intended for.

You can also look for the date code tags.