My quick and probably not perfect analysis of Motobecane (Grand Records mostly) serial numbers.
I only used these 2 threads for this analysis:
Show your Motobecane Grand Record
Motobecane Serial Number Database
1970 to 1975 Most on the rear dropouts.
The model number with year is usually on driver side inside dropout.
Example:
64316 is Model 64 or 6416 (Grand Record), The third number 3 designates a frame build year of 1973,
Model 62 or 6216 Grand Touring
Model 63 or 6316 Le Champion
Model 64 or 6416 Grand Record
Model 65 or 6516 Mirage
Model 93 or 9316 Grand Jubile
The serial number is on outside of non drive side dropout. Seems to be a sequential number that might have started at 1000000 for all Motobecane models.
The serial number might not always get stamped when the frame model number/year is stamped.
(one example I found: Grand Record with 64116/1250892 (Frame built in 1971) but serial number later than Grand Record with 64216/1238406 (Frame built in 1972)
Looks like around 1976 Motobecane started using Month/Year stamps on underside of bottom bracket shell.
Thank you T-Mar for all the serial number databases you have started and figured out