Originally Posted by Blue Order
I think it's incorrect. I checked my frame last night. The stamp was on the lug joining the head tub to the top tube (and another, different stamp on the lower head tube lug). My frame was stamped 75, possibly 76, but it's definitely without question a 77. In fact, I checked the Normandy hubs, and they're stamped 77. The possibilities that come to mind are that the stamp is not related to the date, or that the frame is stamped with the date it's assembled (seems unllikely), or that Motobecane had a backlog of lugs from previous years that they were using up. Your 12 speed indicates an 80s Moto.
Lugs stamped 75 or 76 would definitely blow my theory that it is the lug angle. It's highly unlikely that it is the date of bicycle assembly, as frames are stamped prior to painting, so as not to damage the finish. Right now, the most likely possibility would seem to be date code for the lug manufacture.
I agree that a 12 speed indicates that a Motobecane Mirage is likely 1980s, provided the freewheel is original. If I recall correctly, EGreen also mentioned in a previous post that his Mirage used Motolite tubing, which would definitely place it in the 1980s. An early 1970s Mirage would have used 1020 tubing.