According to numerous sources, including The Aiki News Encyclopedia of Aikido, Michio Hikitsuchi was verbally awarded 10th dan by O Sensei, and later received a certificate from the Aikikai. However, for some reason, it was never publicized.
KAMI : You're right on Hikitsuchi Sensei being verbally awarded by O-Sensei and wrong in assuming that he received a certificate from the Aikikai.
I participate in a discussion on another list about that :
So, as you see, the Aikikai still does not OFFICIALLY recognize Hikitsuchi Sensei's ranking.