This is the original tuning encoder, which is no longer available: Oak Grigsby 94Q 125-02-00606
These are possible alternates, but no verification that they will work:
Bourns ENT1D-B28-L00125L (also believed to be obsolete)
Avago HRPG-ASCA 17R (the threaded collar on the shaft is narrower than the Orion mounting bracket; this looks like it would work electrically but there's one report that it does NOT)
Iwatsu EC202C128A-2TD-0277 (Ten-Tec reportedly used these in later production but it is not known whether the threaded collar is the same size as the Oak Grigsby).