The traditional title went from Master to a more modern title of Professor Bobasan 11/2016
As the story goes from a few high ranking Professors in DanZan Ryu due to the mistranslated Japanese testimony of the trial of Terry Lee a student of Master James Mitose of the Kenpo Jujitsu system the title Master had too much baggage for the Dojos in the states.
In 1953 Terrys teacher was referred to as Master Mitose of the style Kenpo Jujitsu. Terry thought that Master Mitose was mad at a person who owed him money and terry went to collect it and ended up in a murder that was unintended by Master Mitose who was wrongly convicted. The term Master in the trial made Master Mitose sound like he had mind control over his student but Terry was using the title Master as a proper title of respect not out of subservience.
Stopped using Master title and adopted Professor
Along with the suspicion and anger over the Emperor following Japanese from WWII the term Master was thought un American (Noting that Professor Okazaki was an exception and he taught many US service men he was still held in an internment camp in Hawaii). The master term was related to master-slave terms of past American history along with the Terry-Master Mitose murder conviction and In 1969 the Manson Murders,with the reports of brain washed followers, put an end in some Martial Arts to using the older terms of Master and some turned to a modern American title of Professor.
The title of Professor (following Doctor Kano’s Kyu Dan Rank system),usually after 4th or 5th Dan rank comes the title of Shihan (which is also a title for expert or senior instructors it can also be translated as "master instructor" or teachers of teachers).The Yodan or Godan usually is taught all the arts of the entire system and is awarded the title of Professor (now from 5th to 10 Dan). The title of Professor is in recondition of time in the Ryu, continued service and propagation of the system.