That is the double roll pin that holds the Action block (i think thats what its called) in, its present on all sigs.
Knock it out and the firing pin, its retaining block, lockingstuff etc will fall out of the slide.
The slide is folded sheet metal, and all the works are held to the slide by that double roll pin.
Wrong on both counts. One of the holes is filled with a solid pin. This pin holds the firing pin assembly in the slide similar to the models that have a stainless, one-piece slide. There is yet another hole to the rear of the first crosspin and this hole is adjacent to the extractor body. I believe the hole accepts a tool that removes the one-piece extractor, but have had no confirmation coming from Sig on this.
-WF
