here's an '83-84 for sale
http://www.velociao.com/2012/colnago-nu ... 57-silver/
and here's a quote from another nuovo mexico owner
,apparently, was produced '81-85, a precursor to the '86 launched Master
''What Is It?
This is a Colnago Nuovo Mexico, the “new” refers to the crimped Columbus SL tubes. In about 1981, Ernesto Colnago decided to crimp the top and down tubes; something that bike builders had been doing to chain stays for decades. Though it was reported that the Nuovo Mexico was stiffer, there were some complaints of instability at high speeds. Crimping the down tube twice, which conveniently mimics the company’s logo, seemed to solve the problem. And given Colnago an idea for the future.''
http://pezcyclingnews.com/?pg=fullstory&id=9885