I have a post '64 Winchester, like yours. Mine has the typical shallow pitting in the finish. IIRC, the receivers are made of cast austenitic manganese steel, aka mangalloy or Hadfield steel, but plated with an iron coating so that they could be blued. It's the iron coating that deteriorates - I believe the actual mangalloy casting is fairly corrosion resistant.
I have never seen one of these post '64 Winchesters with that type of finish as is on yours. While someone could have had it nickeld with a mat finish, I think its far more likely that it was successfully stripped of its iron coating down to the bare metal. If that is the bare mental, I think it looks kind of cool. I would like to know how it was stripped and would consider doing it to mine. I believe that this type of steel has some natural corrosion resistance - thus why it really can't be blued. The reason why I don't think someone nickel plated it was because the lever is not plated.