I had a model 55 and the tuning shaft would not engage the tuning mechanism.
The rubber "grommet" covering the shaft had shrunk down and become hard and brittle. It would no longer engage the "teeth" of the tuning mechanism.
I went to the auto parts store and took the bad part & tuning shaft with me.
in about 5 minutes, I found a piece of fuel line that was the exact size of the old "grommet" .
It slipped right on the shaft and has worked for close to 15 years now.
This "grommet" is about an inch long. The auto parts store just gave me a couple of inches for free. (A BIG thank You to Cut Rate Auto Parts in Shelton WA).