"What is the difference between a M6 and a M6 classic? The M6TTL I know."
This is a bit of confusion caused by Leica users; when the M6TTL came out, Leicaphiles started referring to the M6 as the "classic" to distinguish it from the TTL. Leica never actually used the "classic" designation. Therefore, the M6 is the M6 classic.
The M6 (classic) lacks TTL flash metering, has the smaller, less ergonomic shutter speed dial, and has a less intuitive LED meter display. Its top plate is a millimeter or two lower, as well. One advantage of the Classic is that its electronics are simpler, and still repairable. By many reports, the TTL has its metering circuits integrated with the TTL functions, and those are on a circuit board that is no longer available. Please, any Leica experts correct me on this if I'm mistaken!