Fascinating. Katori Shinto Ryu is famed also for having a (anti)ninjutsu oral tradition. As an aside to this discussion, I've noticed that some of the very oldest ninjutsu image representation have ninjas holding the hand position in the 7th (Retsu?). If kuji/Juji is not specific to ninjutsu but adopted by them, why does the 7th seem paticulary associated in iconography with ninjutsu? I seem to remember a really old interview where hatsumi is quizzed about this by a young Japanese lady, and her first immediate hand position was The 7th. The works of Gingetsu Itoh seem to emphasise it as well. Any takers?
I think Gingetsu Itoh said that while ninja did use Kuji as well as other magical symbols charms and whatnot they were only for the purpose of self hypnosis and were not really associated with any specific belief per se