I have a Motobecane that I'm pretty positive is a 1977 (based on unique characteristics for that year) and the sn is 3547589. There are threads here that you can try to search out that suggest that Motobecane may have had no code to the SNs, just sequence numbers. And, who knows if they would be continuous or with gaps. If so, that would make yours late 70s to early 80s, later than 77 anyway.
A word of caution about French bikes -- my 1977 came with a metric threaded Normandy hub. Make sure to check carefully before replacing a freewheel.