The plastic knob that used to adjust the seat on my '74 GTV literally fell apart in my hands several years ago. I got some used parts from APE and finally made the repair last weekend. The repair was pretty easy once I figured that the spring clip could be rotated to unlock the knob.
Here's a step by step:
This is how the seat looked without the knob.
The first step is to gently pry off the silver part of the knob. It will come off if you work it all the way around.
With the cover removed rotate the spring clip counter clockwise by tapping the ears with a hammer and punch until you can wiggle the black part of the knob off. I had to "tap" it pretty hard.
View with the knob removed.
Put the black knob on the seat hinge and rotate the spring clip using the hammer and punch to lock it on. I used some WD-40 to lube the sliding surfaces.