Yours is later than the gold one - not way later, but enough to affect the value somewhat. At the time yours was built, and I would guess '77-79, Colnago was farming out a lot of frames, quality was suffering, and Colnago's reputation was taking a hit it would recover from in the mid-80's, when they got a handle on how to do the larger production thing better. The problems seem, however, to have affected the Mexicos less than the lower-level bikes, perhaps because the Mexicos were built with lighter tubing and therefore demanded more careful brazing. Yours is definitely a nice and desirable bike, judging by the photos in the other link, but you maybe never be able to nail the date down further than about a 3-year span. You could take the easy way out and take your cue from the date code on the rear derailleur, until more evidence to further pin down Colnago dates emerges.