Originally Posted by T-Mar
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.
Ok, guy's I gotta fess up to a discovery I made just today. After much scrutiny of the bike, many inquiries, postings, and review of the original pictures, I thought to untape the seat tube and take another look at it for signs of the tubing material, alas, right there where it should be, unfaded and umblemished was a decal indicating 'high resiliency 2040 tubing'
There's a thread on Numbskulls, perhaps I should post this there!