another differences

71s are without sensors on seats or seatbelts. Seatbelt light is controled by the time delay flasher. No buzzer

72/73 gets sensors on lap retractors and on seat bottom springs. Light is controled by a relay on dash which lights on when you gear your car and belts are not fastened. Accomplished with a buzzer

74... the famous/infamous interlock system. Same seat sensors than earliers but seatbelt sensors were changed to female buckles. If still in working order, an unit control box won't allow you crank the engine if belts are not fastened. Acomplished with light and buzzer. Doesn't apply to Canadian models. which also gets a control unit but just to control all warining lights and buzzers, but will allow you to crank the engine no matter if seatbelst are unfastened