That is quite exceptional. Did he ever give a reason for doing this? Is it because he had a need for instructors on a rather short term in order to spread Aikido?
As a contrast, I remember that Ken Cottier (7th dan Aikikai Hombu shihan) never liked to be addressed with sensei - not in the dojo and certainly not outside the dojo.
It was Abbe Sensei's teaching - It never changed. - students were referred to as Mr or Ms.
If anyone came in my dojo using first names they would be out quicker than they came in.
I remember teaching Ken Cottier when he was beginner in Liverpool. He was a Judoka with Fred Wainwright Sensei - Ken didn't have any problems with calling myself and Ken Williams Sensei each time we visited.
I always liked Ken Cottier and invited him as a guest to the `Kenshiro Abbe Memorial Event` in 2005 at Crystal Palace - London.
Co-author `Positive Aikido`