There may be something plated on the case,what it is?

This. 257 Roberts might be covered with something,at first I think it might caused by oxidation or corrosion,but I can’t polish it even by using 1000 mesh sandpaper or weak acids. It still keep its Chocolate-like color,I’m interested in what covers the surface so firmly,I only know
Titanium plated brass cases,copper plated steel cases and Browning’s special black plated brass cases,but about this…I only know this. 257 is made in brass inside. but what covers on it and whether plating treatment or not,this is what I’m interested in.

My guess would ba on a very uniform in-depth corrosion caused by unique environment and maybe chemicals or acids. Means the zinc was taken out in a uniform amount and that well into the depth of the material.
Or the very lot number of this alloy was somehow different and corroded in an unusual way.

But again, only an opinion.

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