Hmmm. "Ya", "To", and "Ei" are fairly common kiai shapes...I wonder what the distinctions in Kogen Itto ryu are.
Re-reading this article today, I see some commentary I did not notice before on the idea that there is no vocalization in Daito ryu. Mainline Daito ryu certainly has that today, you can plainly see/hear it on any of the Kondo Sensei videos, and I think I saw a video of a Takumakai embu recently where they use "To!" when executing the finishing blow.
When I Was training with Kondo there was some basic vocalization at the end of things, but nothing very sophisticated or pervasive.