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.
This, pleasantly moth-eaten, 1956 edition includes a few rather furry pictures of derailleur-equipped bikes, a tiny section actually devoted to derailleurs (including one SunTour and one Shimano and nothing else) and a small selection of quite interesting adverts.
A couple of highlights are:
Browse associated derailleurs.