John,
IMHO, I think it was a handachi modified for wear as a military tachi.
Seldom do we find real tachi koshirae for this kind of sword, too rich to be brought to war. In fact, I have never seen a common tachi mounting, only elaborate ones
http://www.aoi-art.com/sword/wakizashi/ ... shirae.jpg
Even this one is elaborated (not mentionning Efu, Itomaki no tachi much more refined)
In fact, my first katana was in handachi mounting and when you said : I would prefer it to have an aoi-gata tsuba, I was thinking to myself John is reading my mind