DISRAELI DOCUMENTS
It is very hard for non-Japanese speakers, like myself, to uncover the secrets of Japenase derailleurs from before 1970. But the annual 'Japan's Bicycle Guides' shine a brief glimmer of light on the subject. These thick tomes showcased Japan's bicycle and bicycle component industry, and were, mercifully, written in English.
In September 2024 I was lucky enough to visit the Bicycle Culture Centre (BCC) in Tokyo. Amongst many other fine things, the BCC has a magnificent library including every edition of Japan's Bicycle Guide. I was allowed to look through these, but not to make any scans.
This is the 1954 edition.
There is only one derailleur explicitly featured. This is an unnamed pullchain model with a rigid b-pivot shown in a tiny photograph. It is possibly a Sanko S1.