I have not personally seen a number stamped in a seat lug on a Cinelli, although my observations are admittedly limited. I've only seen this on Windsor frames. But this bike sure looks authentic and I have little doubt that it's the real thing.
I have a similar vintage SC with no serial number on the seat lug.
This bike is a bit unusual in that it has a chromed rear triangle and front fork. It sure brought a nice price. I'm curious about other members' observations.
Jeff Pyzyk
Milwaukee, WI