But the modest reel he bought for just 500 yen ($4.40 in today's money) has been revealed as a rare lost cartoon produced by Walt Disney featuring a character that led to the creation of Mickey Mouse.
The discovery was reported by Japan's Asahi Shimbun daily, which Watanabe contacted after reading a book about the history of "Oswald the Lucky Rabbit", the character Disney created in the 1920s.
While Disney created the character, complete with oversized ears and a button nose, he lost the rights to another company, prompting him to develop a new cartoon figure: Mickey Mouse.
The book said seven of the 26 short films featuring Oswald that Walt Disney produced were missing, and Watanabe remembered his childhood reel.
It was labelled in Japanese "Mickey Manga Spide" -- Mickey Cartoon Speedy in English -- and featured a dog policeman on a motorbike chasing Oswald and his girlfriend in a car.
As the trio speed around valleys and along roads, the characters and the vehicles stretched and shrank, the Asahi reported, using a technique that later featured commonly in Disney works.