I'll just throw this out there -- I've heard of others who have success with it . . . toothpaste?
Use a piece of 600 grit J weight back metal finishing belt. The J weight is very flexible. Rub it with light pressure. Do not go in circlesI'd just tell people it's a hard-earned accumulation of grease and gunsmoke. It happens to M12s.
The bolts were not chrome plated, as was suggested.
Use some manner of fine polish to work out the stain, as others have suggested. Chemicals (naval jelly, vinegar, etc) should be avoided as they have the annoying habit of leaving behind their own mark, usually a light gray color that could be harder to get rid of than the stain itself. Know that some stains simply will not come out without serious degradation of the surface you are trying to restore.
My Model 12 Y has the same markings from use in the jewelling. My older pre-64's did the same thing as did my new Browning Model 12's once they started being used. It's just part of owning a model 12. That is just normal wear. To the OP I'd suggest you just shoot it and don't sweat it! Either enjoy shooting the best pump gun ever made or leave it unfired in the box if you can't handle wear marks.