Can someone show examples of these slide differences and or describe them in detail? Also, how has the frame changed structurally, if at all (other than the rail and trigger systems)?
Orginally all Sig slides were carbon steel. They were stamped and folded. This includes the P220 9mm, P220 45 ACP, P226 9mm & P228 9mm. Most of these guns were blued. Some will be found with a K Kote finish some might even have Iffalon which are darker finishes. More Black than Blue.
IIRC the P229 & P239 have always used a milled stainless steel slide. The P220 & P226 are now produced using a stainless steel slide which is milled. These days all Sigs are milled stainless steel slides. They are also finished in Nitron. This is why there is a difference in weight. An older stamped P220s is lighter in the slide than the current production P220s.
I believe that the profile of the P226 & P229 slides in 9mm changed when they started to produce the 40 S&W and .357 Sig models in quantity so they could take advantage of economy of scale when milling. They used to be slimmer but now are thicker so they can use the same slide milling profile on the CNC machines.
Again IIRC there where changes in the height of the slide serrations and there have also been changes in the front grip textures too. There are too many for me to remember which applied to which models.
IIRC the vast majority, in the 95% range, of frames and slides are now produced here in the US in Exeter, NH. The last mass produced pistol that was made 100% in Germany were the SP2022s. As of late last year the SP2022s are now made here too.
Frames have changed from non-railed to railed. Guns like the P220 have gone from an internal extractor to an external excactor again for economy of scale. There have been several trigger variations. DA/SA, DAK, DAO, SAO etc...