If I remember correctly, Winchester changed the gas system to a chromed finish, helping the gun operate better. There may have been other mods done as well but I can't recall all the details.
The Super X 1 was a fantastic autoloader with a milled receiver but it became too expensive to build.
Winchester did not chrome the gas system or the magazine tube. Nuline line guns offers a chrome coated gas system along with a stainless magazine tube which is highly recommended for the Super-X. The highest serial number I have seen on a Super-X was in the
88,000 range yet have read of guns in the 92,000 range. Guns above serial number 60,000 are the most desirable as the newer generation of the trigger group etc were made. Tom Knapp shot Super-X's for his shooting demonstrations before Benelli started sponsoring Tom.....the reason was the Super-X was the fastest cycling auto made. The best auto shotgun made in the USA.