I don't believe these to have stainless slides. All my other SIGs that have stainless slides have the words stainless engraved on the slide. I see no such engraving on these.
I'm not 100% positive you can change this to a DA/SA trigger. I can call SIG to verify. I know it's possible on other SIGs but I've never tried on a SIG with a manual safety.
I don't have any hig cap mags to go with this - sorry.
I was told by a rep from SIG that these were special made for a few departments. That's why the manual safety is there. The manual safety is an expensive option on these pistols. No special markings.
I have 5 in now but can get as many as you want.