I’d have to find a serious history of Japanese armor to answer the question of evolution — it’s the kind of thing some of my students might actually have, but I don’t, though there’s some great detail on helmets and leggings in Conlan and in my Edo Culture exhibit catalog — but I’m pretty sure that I’ve not seen it in the Edo period stuff that I know best.

Thanks for the Kogojiten recommendation: I have one myself (Shogakkan by Kitahara) and, as it turns out, it’s got an annex, too! I could have answered this question myself years ago, and looked so much more clever.

Well, at least I have a great source to add to my scanning project.

If this is the sort of question you encounter often, I highly recommend the 旺文社古語辞典. My 1985 edition has a wonderful annex with page after page of highly detailed diagrams of period dress, all immaculately labeled, each item illustrated in context as worn and then shown in detail separately. You didn’t think I knew this stuff off the top of my head, did you?

I suspect the reason that it stops showing up on the armor — I think I’m right about this — is the bow gives way to the gun and then the peacetime samurai don’t want anything interfering with their swords….