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Nice chart. I didn't see any reference to "MM" though, although I admit glancing over it. Maybe I missed it.
But, I'm guessing the 66 refers to the tip-opening measurement (as others have already written) and the "MM" may mean medium facing length and medium chamber size.
For instance, I have Meyer alto pieces that are 6S-Medium Chamber (which I sometimes refer to as "6SM") and 7M-Small Chamber ("7MS"). The first letter is the facing length and the second is obvious - chamber size.
Hope that clears up part of it for you. DAVE