I think there's 3 types of seat and two types of runners. 68-73 had the adjuster knob for the back rest, then from '74-79 there was a lever for adjusting it, but for 77-79 the runners changed as well. Have a look on the vw heritage site, they've got pics of the 3 shape changes for the seat covers. I think it's 68-73,73-76 and 77-79.
