Unit Speculation?

[O'Hea]

The Odachi pre-dates the Chinese Zhanmandao. Considerably in fact. Japanese great swords inspired the Chinese Zhanmandao. The trouble with the Odachi is that it is not a standard sword. Forms of Odachi have existed in Japan for a long time. (Even before the word Odachi was invented.) Many were offerings for gods, massive swords enshrined in Shinto temples. I've seen some that had blades much bigger than a man.

Are you certain of this? The zhanmadao is shown in a military manual from the 1070's, not very long after the tachi first became widely used. Odachi were obviously widely used in temples or sent as gifts, but these are ceremonial weapons made by smiths as a show of skill, not battlefield weapons designed to fill a tactical role in an army. The nagamaki, on the other hand, appears to have been a combat weapon first and foremost.

A nagamaki is also no more than a shorter naginata as well. Although the quality of their construction would vary. Some nagamaki were made from recycled katanas, (This was true for the blades of regular naginata as well.)

According to wikipedia, a naginata's shaft is normally the height of the user, and ranges from five to seven feet, while a nagamaki's shaft is usually between two and four. One was a polearm and the other was a long-handled sword- this is also why naginata shafts tended to be unwrapped whereas nagamaki were often wrapped in silk and rayskin in the same way as a tachi's would have been.