assume you've already hit the Cadre.org site?
http://www.cadre.org/Merckx/
Probably not the detail you're looking for, but might at least help decoding the number. You used to be able to email or call the Merckx factory and they'd go look it up for you, but apparently the moved to some new system several years ago and most of their historical records weren't included....or at least that was the party line I got on several occasions.
Good luck.....the search is half the fun!