The official Dragon Ball website’s fifteenth entry in “The Nearly Complete Works of Akira Toriyama” — an on-going series highlighting rare and important pieces of the author’s work over the years — is a sample from the second Bird Land Press release — the Akira Toriyama official fan club newsletter — from September 1983, with characters from “Ageha (Swallowtail Butterfly) Town Observation Diary”:

Toriyama-sensei comment: The title’s “Ageha Town Observation Diary”, and I drew it before “Dr. Slump”. It’s a phantom work that got rejected by Torishima-san!! There’s a kid with wings in it that became the basis for Gatchan. I really like winged kids like this, and I kept drawing them in previous works. And I was really gung-ho about making it the main character in this one… Anyhow, this work became my basis for creating “Dr. Slump”.

The official website’s writeup briefly details this same origin story, including some of the direct connections to Dr. Slump such as Gatchan, and its ultimate fate of being rejected:

The characters of “Ageha Town Observation Diary,” a comic drawn before Dr. Slump. Because it ended up being rejected, it’s an extremely rare work that wasn’t made available to the public anywhere else. Ideas that would form the basis for Dr. Slump, such as the little one with wings, are present in abundance.