Genuine early Mexicos are pretty rare, though of course as soon as one sells for big money on eBay, others are sure to follow. Later, they were all but indistinguishable from the Supers. Early ones reputedly had very lightweight tubing - supposedly Columbus Record in at least some cases. It's open for debate whether they were actually built using that tubing, since it's really too light for production frames. I think what's happening in part is that the flow of early and mid-70's Colnago in general is beginning to dry up, and prices have been moving upward. At least some of the early Mexicos also had a special Campagnolo group as well - which would be a real chore to put together if all you were starting with was the frame. How early is your Mexico?