Nitro is a salt base finish, like parkerized. But it is finer, so it looks nicer.
The finish is durable and looks really good. I have it on my P250, P228 and P239 they are older models, specially the P228. it has a roll pin for the slide, so you know it is an old model.hehehe.
But if you want the official answer, call SigSauer and they will will tell you.