Yes the sleeve or double butting we all came to the conclusion is to reinforce such a extra thin seat tube. I own and have seen a lot of early colnago's super, mexicos and masters so I can see it is not likely to be a fake. The size of the seat tube and I cm thinner gives rise to the narrow seat tube diameter. Normally 27.2 ... not sure what steel it is columbus aelle has 26.8 but would not be racing class although i have read some slx tubing of that time may have smaller diameters. What is interesting is the groove on the colnago motif on the chain stay, normally for the mexico's the groove is on the inside. It also has a slight inside groove.

I am not sure if colnago sold parts seperately so someone could assemble such a bike then? But then the BB is a one off with the genuine colnago panto . But it could be something that was assembled like you say a design for a particular racing purpose try out. It seems to have a mix of all late 70 early 80s types..

The paint looks like a team aftermarket colour paint sponsored by a particular shop thus not sure what it might originally have been, not an original colnago paint job . The BB with the true colnago motif with the extra sleeve is a definite one off, so someone must have made it especially for that tubing mixture.

Would love to know how it came about and for what purpose..