stainless sigs
Further compounding the issue is that a number of SIG models had stainless slides that were anodized or finished in some fashion, so that they did not appear "stainless" but blued. To the best of my knowledge, a stamped, non stainless, P220 slide will have the dual, hollow roll pins and the modular breech block w/ integral extractor, but it could be nickel or dark finished.
A stainless slide will have a single, solid pin, a modular external, extractor
assembly, but could be in the white, or dark finished.
There were certain models of the P220 that have solid, machined, stainless slides, I believe they were billed as match or comp guns. (I think)
The P226 .40 is an example of this arrangement, I have worked on those.
But they are dang heavy for a duty gun.