As the engine number doesn't match the frame number, that also indicates a likely replacement. The engine number indicates an 11/1980 production date (1981 model year), so I think it could be that somewhere in the bike's history, it could have had an engine and tacho transplant with the original 3/1979 production frame.
Definitely not a 1984 model, anyhow, though many, many parts are swappable/interchangeable.