If you have never put covers on before have an upolstery guy show you how the first time.When you take the covers off the 73 seat you will find a wire in pleat of cover that is used to pull cover tight.In the 73 up back the wire will only run vertical 1 on each side.In 69/72 the wire also has a horizontal piece that connects the sides together .Without this horizontal piece 69/72 covers will have sags.It is only a small piece of wire but knowing how to adjust and tighten covers will make a lot of differance in the final apperance.
