Recently purchased a two tone P220 in excellent shape. Came with 2 dove tailed 7 round mags in an non original gray plastic Sig box. All controls including grip screw and hammer/trigger are nickel. Serial number is G260 xxx. No two letter date code. The slide is marked "MADE IN W GERMANY" wit the "W" marked out.

I prefer a brushed stainless finish as opposed to a bright nickel finish. This 220 has a very dull, gray finish. How thin is the nickel finish? Would polishing the finish give me my desired look? Or should I just leave well enough alone?

Leave well enough alone, unless you're sure you know what you're doing.