Functionally my M6 was identical to my MP. Everything looks and acts the same. The only difference is the film winder, and film rewind. And the ISO dial - which is broken on mine!!! It cannot be set at 400 for example, only above or below!
When I compared both, the VF is identical. The meter is identical. Everything you can see in the camera is identical.
When Leica was actually asked - what are the new bits? No answer. Now think about it logically. The MP is a current production item. The M6 works exactly the same way. Why would it make any sense for them to have different bits inside the camera? If there were changes, then the MP would be changed too. From a manufacturing and economic view point, the more pieces that are interchangeable across the line, the better it is for efficiency and the company's bottom line.
Again, when asked what is the difference, Leica only points to the original M6. Not the MP. And the new M6 is different from the original one, as it has the meter from the MP, brass top and bottom plates (instead of zinc), it has the VF and RF mechanism from the MP which is much better than the original M6 one.
They are intentionally vague and ambiguous, as they wanted to hype it up. And not have people think 'oh, it's just the MP but w a red dot now'. It's all about marketing.
The ISO dial on my new M6 - trying to set it to 400: