Just to add to the story: I have a 1991 Colnago Super SL that was a professional Belgian team bike - 'Collstrop Isoglass' in the original colour. This team only used these Colnago's for the one year - 1991 so I have no real problem dating the frame. My only question could be that the frame may have been a 1990 version enabling the team to be 'up and running' for the 1991 season. originally it had a Suntour Superbe Pro groupset.
Notice the rear brake cable is routed through the top tube and both entry and exit holes are on the top of the top tube.