The above URL links to an explanation of the old certificate system used in Japnese martial arts. (Menkyo Kaiden) It's written by Tiki Shewan, a 6th dan who was uchi deshi to Tamura.
O Sensei, according to Kissomaru, "mastered" two different jutsu styles, and also studied judo for a short while. He was probably the top student of the Daito Ryu revival, although I read an interview with a Daito Ryu sensei claiming he had not actually recieved the final certificate. (The final certificate in the Menkyo Kaiden system corresponds more or less to eighth dan..) I believe he held certificates for sword and spear also, but I don't know which certificates. (The lowest certificate in the menkyo kaiden system is roughly equivalent to 4th dan.)
In December 1940, O Sensei "suddenly forgot every martial art technique (he) had ever learned." We're not in a position to really understand that kind of revelation unless we happen to achieve mastery of the art, I suppose, although I'm sure people will disagree with me there. (I may quite easily be wrong on some minor points about his life)
My point, I guess, is that O Sensei held a number of "high" grades, and none of them matter. Anyone in a position to question his credentials is unlikely to be interested in doing so. And for our sakes, "founder of Aikido" is plenty.