Here's a very interesting bike:

http://www.renaissancebicycles.com/g...m=10&gallery=3

Note that it has "Colnago" on top of the BB shell like yours does. It's interesting because it does not have "Colnago" stamped in the chainstays, like yours, which makes me think it's a bit later - say, post-84ish. But it has the channel in the top tube only, which doesn't fit any models I'm familiar with (and differs from yours in that respect). Text identifies it as a Superissimo, but it could easily be misidentified, especially as it's repainted.
Is it possibly a Superissimo, and some Superissimos had, for probably a brief period, a single channel in the top tube? Sure.
Is is possibly a Nuovo Mexico that only got a single channel for a particular run of frames? Sure.
Is it possibly 1983-4 and some bikes didn't get the "Colnago" in the chainstays? Sure.
My hunch is that it's 1985-6, and that the "Colnago" on the shell made its appearance later than the clover, which would also explain the missing "Colnago" in the chainstays. As to what model it is, I think it could be either a Superissimo, as stated, that made a fairly brief appearence, or a Nuovo Mexico that simply lacks the downtube channeling. I lean toward Superissimo, but it's definitely one of those 'tweener Colnagos that makes the brand endlessly fascinating and frustrating.