The IJ Champion Mdl was made from 1909 to 1948, and should be in the smokeless powder era (patent dates will always be earlier than the date they were made) However, 12 g shells were not always 2 and 3/4 inches long- as the gent above me noted- early shells were 2.5 inches. DO NOT assume that just because a shell will chamber that it is safe to fire- the shotshell opens out into the forcing cone- and excessive length of shell equals VERY high pressures.
Good idea to have this one checked by a gunsmith before shooting.
IJ specialized in making low cost, reliable, working man's guns. In general, they do not have a very high collector's value, which WOULD be damaged by "cleaning up" your shotgun. In other words, feel free, it will not hurt value.
Do not know about "nickel"- these were blued steel, with a color case hardened steel receiver. Most of the bluing and color case hardening is gone due to age and honest wear. Either 0000 steel wool and light oil can be used, or just light oil on a clean cloth, and a LOT of elbow grease. for the stock, Formbys Refinisher (be careful highly flammable) and some Tung Oil or Birchwood Casey oil finish. Yeah, brasso the brass- a coat of neutral shoe polish (kiwi) will help delay it tarnishing.
Old Western Scrounger was one source for old time shotshells.