OK, folks, prepare to be pissed off.
Back when the few agencies in existence created their versions of Master Diver, or whatever you want to call it, scuba diving was in its infancy. Only a few agencies were teaching only a few classes. If you got the "Master Whatever" rating, you had pretty much exhausted the instructional possibilities. There was no real technical training going on. There was no cave training going on. People had no idea what they were doing with wreck training. Equipment was rudimentary compared to what we have today. Our knowledge of decompression theory and deep diving mechanics was just beginning.
So a person who achieved the master diver rating back then had scored a major certification, something to be proud of. The requirements for those ratings have not changed in the half century since they were created, but the world of scuba has changed dramatically during those years. Thus, to many people, reading posts about whether one agency's "master" rating is superior to another reads a lot like arguing whether an 8th grade education or a 9th grade education demonstrates true mastery of academic content.