Osomatsu-kun (Found Anime Series; 1966)
Osomatsu-kun (おそ松くん) was a Japanese animated series from the 1960s and 1980s, centring around the comedic misadventures of the Matsuno sextuplets and other strange characters such as Dayon, Dekapan, Totoko, Chibita and Iyami.
Based off of the comedy manga by Fujio Akatsuka, the show was very popular during its time and reran in both Japan and South Korea. There are three anime series in the franchise. Two of them bear the Osomatsu-Kun name, one from 1966 (in black and white), one from 1988. The latter focuses more on the characters Iyami and Chibita rather than the Matsuno brothers, but it was still considerably popular.
By far the most popular was the 2015 anime Osomatsu-san. With a bold debut episode that mercilessly mocked numerous other anime series (and even had to be removed from international streaming and the upcoming DVD release due to issues in Japanese copyright law regarding parodies), the show gained a whole new fanbase, many of whom are quite eager to seek out the older series.
Due to poor storage of the original video and audio (similar to the first anime of Doraemon), it was believed that the 1966 anime was entirely missing from the archives sometime after the 1970s reruns ended and could never be re-broadcast in the decades following. However, 16mm prints were discovered at a TV station warehouse in 1990, and the 1966 series was again able to be broadcast on television and released on video.
The Osomatsu-Kun series from 1966 is now relatively easy to find - it was released on both VHS and DVD, and multiple volumes can be purchased through Amazon Japan. The 1988 series was released onto a large DVD box set in 2003, but it is currently unavailable for purchase.