But the standard make-up is a milled stainless slide, Nitron coated atop an aluminum alloy hard anodized frame. DA/SA or DAK triggers. Frame mounted decockers. Newer versions have grips without grip screws, older versions secure the grips with a typical 2 screw arrangement.
Pretty bulletproof pistols. A guide I have always consulted when I look at used SIGs is searchable under "Florks Guide to Sig Wear". I won't copy it verbatim here, but it is worth the read. In short, look at the rails; they will progress from dull black (no wear), shiny black (just barely wearing in), dark orange (just getting into the anodizing pigments), bright gold (wearing in the anodizing), light gold (wearing through the anodizing; need to keep pistol well-lubed from here out), shiny silver (anodizing starting to go), dull silver (anodizing worn through - gun is going to need replacement). Note that a scratch or ding through the rail anodizing that shows gray doesn't mean the gun is shot, you want to look at the full length of the rails.
After that, the top of the trigger disconnector should be radiused, not worn to a flat spot. This is easily replaced, but would indicate a lot of miles on the pistol without the right lubrication. And speaking of lube, Flork has a Sig Pistol Lubrication Guide out there on the web, too.
I have a Sig 229R in .40, but have both .357 Sig and 9mm conversion barrels for it. Great gun! It is both accurate and 100 percent reliable. It is bigger and heavier than my current carry pistols, but great none the less.
I currently own 6 Sigs with mostly them all being of German manufacture, but have owned the newer US made ones. They are quality pistols and have a great SA trigger pull. More info on your trade would be helpful. Also, pics might help as well.