Should be on one of the rear dropouts.
Those probably aren't even serial numbers. No one has been able to figure out what they are exactly.
OTOH, at some point in the 80's (if memory serves) Colnago did begin using serial numbers, so a late 80's, early 90's frame may have one. I just don't know where there's a data base that would make sense of it. Note that those numbers start appearing on dropouts by the early 70's at least. At that time, it was common for Italian frames to dispense with SN's - De Rosa and Masi, to name two, didn't use them either.