There are older SIG-Sauers that were assembled in Tyson's Corner, VA. My understanding is that those should have used German manufactured slides. The Tyson's guns are not 100% compatible with newer make P228s, and when they die or wear out are generally sold off as parts to people who want to keep their own Tyson's SIGs running.
In the SIGarms period, most of the classic P-Series guns would have had frames built in the US and the upper parts made in Germany, with the whole gun being assembled in Exeter, NH.
The general comment, though, is that there are a lot of different manufacturing variations of the P-Series guns. All German made and proofed versions with matching serial numbers are more valuable than run of the mill guns. Non-rail P228s are generally sought out by particular P228 aficionados.
If you have a serial number, you can get started in
this thread on SIGforum to try to learn more about this particular pistol.
ETA: And in following up on my own link, the P228 may not have been one of the guns made at Tyson's Corner. There were some earlier ones assembled in Herndon, VA, though.