What is the trick to remove the plastic trip odometer knob on a 75 914???? I don't see any screw?

Very small set screw on the knob.

We're the late ones plastic? I thought they were all steel, but I don't get as much exposure to original 75/76 cars.

If it is steel, the best way I've found to get those troublesome little screws loose is to find a good fitting screwdriver. Not close... Perfect fit. Then grab a small piece of sheet metal (flashing) to use as a heat shield and one of those torch style lighters. Carefully apply heat and penetrating oil (not WD40) a few times. Then heat once more and try to unscrew.

I want to add to this thread I repaired mine today and it had the plastic knob not the metal one with a small set screw on the side. I put two small screw drivers on the side and slowly popped the cap off. It goes back on and has a flat side that has to match the cable end. Repaired my odometer using the pelican tech site.

All 75 /76 cars have the black plastic knob. Do what gasman stated and it should come off.