Great find. Sig still makes guns with carbon slides, but they are all made in Germany, and you have to import them. A quick way to tell is to look for the pinned breech block. I have 2 recent 228's made in Germany with carbon steel and neither have the "trifecta" serial numbers. I believe this stopped in late 2001. If you have the stamped German proof marks, and the triple serial number I would take a guess that it is a 2001 manufacture date.
You can still get fully German made 226's with a rail. I know of at least 2 users who purchased them in the last 4 months. I would have to check, but I don't think either came with German proof marks, or trifecta serial numbers.
Sig 226, Sig 228, Sig 225, Sig 229, Sig 220, P6, Rem. PSS .223