In an interview with IR, Justin Stailey from Leica explained how future M cameras will be labeled: Leica M will be followed by "Type xxx". For example the new Leica M is Type 240, the Leica M-E is Type 220:
Q: Now on to the cameras themselves. You changed the names of some of them. You moved away from model numbers. So going forward, it'll just be the current M.
A: Well, the model on that naming strategy comes from the automotive industry. If you look at Porsche, they make a 911. But to a 911 owner, it's more specific. From the early 90's, there's a 964. The last generation air-cooled is a 993 from the end of the 90's. Then there's the 996, then 997, etcetera. So internally, they have project names or product names. But to the world, it's a 911. We're trying to go to that sort of a mentality, so it's all an M camera. To be specific, the new M is really a type 240 and the new M-E, a Type 220. That's how we plan to differentiate them. And they're actually marked on the cameras as such.