This is from the 66/67 Charger source guide site-
M2 = This is not an "option" on the car, it stands for the car was made on assembly line #2 at the factory. There were 2 assembly lines for the 67 Chargers & Coronets and if your car was assembled on "line #2", it was coded this way.
Not sure if it's accurate or what their source is.
Source Guide
