Recycled Warranty Cards

Probably legit. Please keep in mind (there are many, many posts and exchanges about the matter...) that there was no such thing as a "Model Year" at Ferrari Maranello at the time; only Ferrari North America was using the "Model Year" criteria; so FNA probably received too many warranty cards, printed in Italy, for the M.Y 1980 (U.S cars had a different Warranty Card than ROW...among differences, the only one with the "M.Y" designation), and probably extended their use with a sticker over the "M.Y 1980" for the "M.Y 1981".

Stickers in the "official" Ferrari manuals are not uncommon; for instance, during the last year of the carbed GTBs production, many european owners' manuals had a small sticker inside the manual over the engine power characteristics, given until then at 255 hp, but covered with a sticker that says "230hp" when Ferrari started to quote engine power in DIN figures, not SAE anymore. The sticker spared them an updated edition for a complete manual for the few last cars.
Rgds