The mystery of Japanese Moving Coils

I have long been fascinated (I must lead a rather narrow life!) by the origins and design heritage of some of the great MCs from Japan.In recent years I have noted a sort of 'coming together' where the sound of some leading cartridges become more alike...as a simple example, the 0.6 Koetsus sound , to my ears, a bit like Dynavectors. Is this all co-incidence or is there a similarity of source? By which, I mean are several apparently different brands being OEM manufactured in one place? The origins of many of these wonderful cartridges seems shrouded in  mystery...does anyone have any information on this? Or are all the various manufacturers really building their own cartridges (and even then, using commonly sourced components). I would love to know!