The original version of 'Empty Socks' was five and a half minutes long but between 30 and 60 seconds from the middle part of the footage found in Norway are missing.
The historic piece of reel has since been authenticated by Disney cartoonist David Gerstein.
The cartoon features Oswald, the Lucky Rabbit, a predecessor of Mickey Mouse who appeared in 26 movies by Walt Disney and his countryman Ub Iwerks.
The National Library said it had digitized the short and sent a copy to the Walt Disney Company.
Before this find, the only copy of 'Empty Socks' known to exist was a 25-second sequence preserved at New York's Museum of Modern Art.
The Norwegian film originally belonged to a private individual, before ending up in the collection of the Norwegian Film Institute - which handed over its collection to the National Library of Norway.
The majority of the library's works and documents are stored in a high-security bunker in Mo i Rana.
According to Norwegian newspaper Aftenposten, the nitrocellulose reels - a highly flammable compound - are stored in a room where temperature and humidity can be controlled.
The National Library has launched a large digitization project to make its collection more accessible, which has helped discover lost documents.
Along with Empty Socks, the library also found a copy of another Oswald cartoon called Tall Timber, Huffington Post reports.
Made in 1928, it's one of the last Oswald films made by Disney and Iwerks, who lost the character in a contract dispute to Universal, which had been distributing the films.
That ultimately led the pair to create Mickey Mouse.
The Walt Disney Company eventually took back the rights to Oswald in 2006 in bizarre deal with Universal.
Sportscaster Al Michaels, then under contract with Disney-owned ABC, was allowed to join Universal-owned NBC in exchange for the character.
- The Daily Mail
