[DuMont]

I'm sure a lot of others have come up against this issue in working on a magnetic tuning system.  There is a very small plastic piece that insulates the spring contact on the bottom in this picture from the shaft.  Without that piece, the radio is silent.  When that spring is depressed by the magnetic tuning dial yoke, it cuts out volume as you go around the dial to your preset station.
So...
No one apparently makes a replacement for the piece in question, shown second from bottom.
I took a bit of 3/4 inch heat shrink tubing and cut it to the proper width using my broken piece as a pattern.
Then I asked the wife if I could borrow her hair drier....
A few passes, testing for fit in-between, and the cut piece of heat shrink slides snugly onto the magnetic tuning shaft.  
The heat shrink is shown third from bottom
Finally, don't forget to put in the  fiberboard insulator (shown at the top) on the side of the spring contact facing you before you put the whole thing back together again!