Clearcoat on Clutch Knob?
 

I'm in the middle of cleaning my very first sewing machine, a 1928 Singer 66-6. In working on the hand wheel, I've noticed some spots around the edge on the face of the clutch knob that look like clearcoat has worn off. So, my question is whether Singer did put a clearcoat on chrome parts. Or, maybe they didn't even use chrome then, and it's just polished steel with the clearcoat to protect from rust?

If it is just a coating of shellac, that should be easy enough to restore after polishing. I'm just being extra cautious in my first project.

Hmm. Well, if the clutch knobs were shellacked, I've polished it off all of mine.
You may be seeing dried oil, perhaps from the fingertips of the last owner?
I'm not positive, but I don't believe there was a clear coat. All the info I've seen talks about using a metal polish to clean up shiny parts.

No shellac on the chromed parts, at least originally. Chrome is hard, durably and doesn't tarnish, but in some cases it can start to flake off, and in extreme cases even wear off. Keep the machines in a dry heated room and the chrome will stay fine. You can polish it, and the metal under the chrome layer will shine up pretty well too. Early machine have nikle plating, and later have chromium. Nickle has an ever so slight yellow tint to it, while chrome are almost towards blue, at least in comparison.