Thread:GreenMoriyama/@comment-28254399-20180203215556/@comment-3391671-20180204014540

You are very much entering old Japanese history, which I too am not fully knowledgable in (nor is any normal native Japanese person). Maybe in a parallel vein to old English, old Japanese is a monster of its own.

Anyways, I'll bring this topic to my colleagues at my Uni (UC Berkeley), as they have likely encountered this before. However, from my current pool of knowledge, I wouldn't go as far back as old Japanese, as these names were likely made from only the modern interpretation (though this suggestion contradicts why other names have their old language etymology... so tell me if you still want to put them in).

Hardened "y" wouldn't have changed to "du" as I don't have any examples in mind of Japanese letter changing like that. There have been cases of the consonant falling off 思し->思い (not the best example, even for myself, but just one to offer what I mean), but none as grand as hoping consonant lines.

As for tu & du, take it that Japanese has always been an oral language, and tsu and dzu always existed before. Tu & du were simplified into being used along side tsu and dzu because of what some would call the softening of the language. Yet again, I wouldn't go as far as point this out on the wiki page as old Japanese was likely not considered into the naming conventions at all, as they differ from Modern.