The original knob is bakelite, an ancient form of plastic. Unlike plastic, it avails itself to repair by filing, sanding and polishing.
The replacement knobs are not the right color.
I restored an antique turn signal which consists of a large round body made of bakelite. It was a mess from oxidation. I was able to bring back its original luster by using sandpaper from coarse to 800 grit. I then polished it on a bench mounted buffer.
Holy Moley does that thing pop now. It has a tortoise shell look to it due to the marbling in the bakelite.
