Here is an option........Reverb has a listing by "The Bonfire Vanities" in Portland, Oregon. These appear to be the correct knob for early Sunn amps (correct shape, numbering, set screw and numeral typeface)....except that the aluminum insert has been painted red. In the past, I have restored Sunn knobs with corroded inserts by polishing the aluminum insert with a Dremel tool, small buffing wheel and some polishing compound.
I imagine that the chipped red paint on the listed knobs could be buffed off. Just a thought. These knobs are a bit spendy, but might be a good solution. No affiliation ....just trying to help.
Ryan