The knobs on all three units are dirty. I tried washing the worst looking one from the DX-60 by hand and normal washing detergent. It helped, but not as much as I had hoped. So the guinea pig got a ride in the dish washer.
That helped a lot. It is now also clear that someone tried to glue the cracks before. I will give it another try. My bet is that all the knobs will get replaced.
This style of knob is used on many Heathkit products.