I don't think there was a cover for the lamp. My gran's machine didn't have one, anyway. Mine doesn't either. I always thought it was funny that it had the little lever to lower the bulb; when I was a kid I assumed that was so you could get the light even closer to your work and I liked to sew with it lowered. Of course that just put it right where it could burn the back of my hand, LOL. (I was a cute kid, but sometimes not the sharpest tool in the drawer...)
I don't think that looks like nicotine stains either - I'd expect those up on the arm and head rather than down on the deck - seems like it'd be constantly buffed off of the deck if that's what it was.
I'll be really curious to see what it looks like when you get whatever that is off. This might be a good candidate for a repaint! I've never seen a repainted 332, could be a very unique machine, lol.