Probably going way off topic here, but outside of Ellis' comments, has there been any looking into the use of the
symbols and their meaning predating Ueshiba's use of them? For example my own ryu (dating from about 1600) uses all three together with an additional two (those being a straight line and basically a 90 degree "L"), the names in Japanese being 方圓曲直鋭, with the symbols matching as 方
,曲 "L",直"l", 鋭
. If I recall correctly Karl Friday's "Legacies of the Sword" discusses what seemed to be similar terminology of five (without showing the symbols) as well. Does Ueshiba's usage of these symbols generally follow the standard as they appear in different arts? And is there any sign that Ueshiba at some point used all five? All of my books relating to Aikido are on the other side of the planet so I don't have much to go on in the library off hand.
Adding to the drift,
One problem is that Ueshiba explains the symbols in terms of kotodama theory. He uses the the circle, square and triangle, but also three more: cross in a circle in a square; circle bisected four times; same bisected circle in a square. This is all in the (as yet untranslated) Takemusu Aiki
volume. Clearly kotodama theory would be the place to look.