There is another number below the serial number that cannot be read in the photo. There is another stamped number, looks like 018. It is all original except the tires / tubes. I have a 1978 Motobecane Mirage, been in the family since new. If you look at other photos, you'll see that Motobecane did use the flat topped version of the number. A '3' would be in sequence with the other submitted serial numbers. The lead character actually appears to be several partial stampings of a '3', (see attached, highlighted photo). Current data suggests the serial numbers are purely sequential with no year designator and a '7' would be greatly out of sequence. I do not believe the lead number is a '7'. The numbers stamped on the drop outs are 47917. The bike's serial number on the bottom bracket is 7749062 followed by 702.

It can sometimes be difficult to nail down exact year of manufacture on these things as they rolled over both old photos and specs in some catalogs, and components may have fluctuated a bit from catalog specs depending on availability. The French made models are pretty old and with the exception of a couple of models (the Jubilee and the Tour de France) aren't worth much. They are both made in Asia but have different features. There are 2 Motobecane versions currently sold now- one is sold only in Europe and one sold only in North America. Comparing the catalog photos and spec sheets, I determined it is a '77. I've checked out those exact sites back in 2009 when I originally posted my photos of the Super Mirage.

Motobecane serial number decoder