Howdy, again.
I had an old, dingy SR stem in my parts bin, so I decided to go at it with 600-1000-1500 grit paper
SR_1 / SR2 / SR3

I thought it didn't come out horrible, so I took my CRS emery clothe to my Cinelli stem gouges (then MED, then FINE, 600, 1000, 1500).
CIN1 / CIN2 / CIN3 / CIN4

The gouges are pretty much gone. There's a faint one left on the front of the stem.
But as you can see, I took off some kind of coating. I don't know if I sanded off anodizing or just a normal patina from 20~30 years ago.
Anyone know for sure?

Regardless, closing the loop on this question, in case anyone was curious.