Frederic,
I'm guessing 1979ish . Clues: spoon stay caps, cable guides on the top tube, derailleur guides on the top of the bottom bracket and the flat crown fork. Later models would have the flat stay caps with "colnago" stamped in them, cable routing under the bottom bracket, sloping fork crown with a different logo on top. Earlier models also have clamp on brake cable guides, earlier yet have almost no braze ons (shifters, water bottle). It's also a definite repaint, with all the chrome painted over.
I also believe this to just be a Super, not a Mexico. See Mark Bulgier's scans for early 1980's catalogs:
http://bulgier.net/
Gary Watts
Vancouver, Washington
USA