The knob is actually several parts: 366-1385-00 (or 366-1385-01+366-0293-01 for SN B090000-B142149) is the outer grey knob, and 354-0413-00 is the transparent skirt. So it makes a difference what the serial number of your instrument is.
Sadly, according to the RPR-knobs&pushbuttons, these were only used on the 485, which makes finding a spare one unlikely.
How much of the knob plastic remains on your knob? (not to ask an indelicate question)
It might be possible to make a mold from the existing plastic and fashion a patch from the casting. This is something that I have been experimenting with, though I have not yet struck upon a method that produces perfect results.
-- Jeff Dutky
