Colnago are not my area of expertise but during this period most other brands used a BB shell with stiffening rib between the chainstay flanges, on their SLX frames. This eliminated the need for a traditional chainstay bridge. Your bicycle has this style of BB shell and the 1988 literature makes reference to a "ribbed" BB shell. The lesser models do not use this shell according to the literature. So this one feature would seem to imply that it is a Superissimo.

Of course, the true indicator is the presence of the splines in the BB end of the tubes. I have heard of cases where careless builders have cut from the wrong end of the tube and removed all the splines, but too have this happen on all four tubes would be too much of a coincidence. Still, they can be hard to detect, as they are only a fraction of a millimetre in thickness. A close inspection with a strong light and magiiifcation is warranted. Good luck.