Recently purchased a German Slide P220 on an American Railed frame. Works well together and shoots great. Just wondering if its a frankengun or a Sig rework of some type?

Serial number only on slide: G 358 5XX

There is no way to know for certain, since you are not the original owner. But, SIG did release folded slide railed P220 in 2004 that was assembled in the US.

Serial number "only on slide"? I think you mean frame. The frame has to have the serial number. Every other location is inconsequential.

I had a 220R pinned slide on railed frame. Traded into it in 2006.

yes, serial on frame. Was told it was a pre redbox by the seller. I got it for a song and it is really tight slide to frame and shoots very well. Actually very acurate too

Thing that was throwing me was the Made in Germany Slide stamping