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Looks like it was bead blasted then cadmium plated, to me.
What he said...looks like "white" cadmium plating (as opposed to the 'yellow' cadmium you see on headlight actuators, vacuum relay, etc.) was applied after the castings were run through a tumbler. Very nice-looking finish...you oughta go back to the same store and see if they have other ones finished the same way. I'd be surprised if that was an aberration...it's definitely not chrome. Certainly not NCRS correct but pretty nonetheless.
I just purchased some of these too...i ended up hitting it with some acetone and just painted over to match my other calipers. They looked like they were cheap knockoff calipers because the name/letters on side of caliper werent there.
If you think this ones wierd......one of the replacements I recieved was a gold version of yours in the pictures and the other was a factory gm rebuilt caliper that looked like it was hit with a wire wheel, it was clean but nothing lik the gold/silver ones ....the 3rd caliper I replaced was silver looking like the one in your pics.
i have a pic of the gold one on the car ill dig up for yall....has to be some kind of plating since one of mine was silver and another gold.