This flies in the face of everything I've read and concurred with Leica experts over the years. The actual denotations, at least as I've come to understand them, are obvious and confusing at the same time.
The "3" in M3 had nothing whatsoever to do with those screw mount models preceding it. Rather, the "3" in M3 referenced its three framelines (50/90/135). The "2" in M2 was a reference to "less than" M3 although it also had three framelines, as the M2 was a less expensive "entry-level" model. The "1" in M1 was "less than the M2" and had no rangefinder focusing. The "4" in M4 was a return to the frameline reference (the M4 had 4 framelines - 35/50/90/135) and was the next level-up above the M3. The "5" in M5 referenced the "next level" - or order-up. The M6 had 6 framelines (you get the idea) and was next in-line.
From that point forward the number references the next model up - until the M240, which was to start a new M numbering system, but Leica abandoned that.
