Very curious about this detail: that does not appear to be a "quill with expander wedge" seat post but the bolt isn't the bog-standard set-up that pinches the top of the seat tube...so what's going on?
does the bolt fit into milled grooves in the body of the seatpost? Or some other "too-clever novelty"?

as you say given it has Columbus DOs and Ital BB chances are high that this is a Titan frameset made in Italy (and not Switzerland) so probably is Columbus SL (maybe Cromor) but check the butt of the steerer as well as the butts of the main tubes and stays inside the BB shell for spiral "rifling"