I was reading somewhere that some karate practitioners do not consider the American version of Isshin-ryu legitimate because the GI's that learned it in Okinawa got their blackbelts in less than a year then came back to the states and started teaching. There was even something about how Shimabuku gave some people higher dan ranks with the expectation that they would grow into those ranks. Instead those people started to use those ranks immediately upon returning to the states.
I think this basic set of statements are more-or-less true. Keep in mind that I am a simple student, a brown belt (san-kyu) in Isshin-Ryu, but this is what is stated openly in my dojo.
However, that is not the entire story.
The various 'first-generation' students were also visited by Master Shimabuku years later, and he approved of what was being taught, and as I understand it, reconfirmed the advanced ranks of the American senseis whom he had taught and given advanced rank to.
As well, some students spent more time with Master Shimabuku than just one year; Master Mitchum being a prime example.
My question: what makes a style legitimate?
I beg your pardon, but that was not the question you asked in your subject line. You asked if Isshin-Ryu was legitimate. My answer is, yes it is.
In the first paragraph of your statement, you insinuated that American Isshin-Ryu is not legitimate. My answer to that is that yes, it is legitimate.
If what you are asking INSTEAD is an entirely different question (what makes a style legitimate), then my answer is 'general acceptance' in the case of practitioners, and recognition by a body that itself is recognized, such as governing bodies.
If those American Isshinryu practitioners took what they learned then researched new bunkai to the Kata; bunkai that works and makes sense, then isn't that as legitimate as any other style?
Bunkai is not what defines the style, as I understand it. Many Isshin-Ryu practitioners have their own bunkai, which they either came up with on their own or learned from various instructors. However, if they belong to a particular organization (mine is UIKA), then they teach the kata and the base bunkai that the organization dictates.
Although there are many Isshin-Ryu organizations in the USA and even in Okinawa, I have not seen that much variation in Isshin-Ryu techniques or teachings that would make anything I've seen utterly alien to me. We all use the vertical fist, the snap kick, the same kata - even if practiced slightly differently from place to place. I honor and respect them all.
As to the what I *think* you may be asking, which is the legitimacy of Sanchin-Ryu, as the founder of Sanchin-Ryu was at one time an Isshin-Ryu student? I have no comment other that to say it is not Isshin-Ryu. Is it legitmate as a style of its own? Sure why not? That's not up to me to say, however.