yep: i agree a very nice looking frame and the guess that it's a 1980 (per serial number) also good since that's right in the "sweet spot"for a portacatena Campy DO and also the transition from nutted brakes (as this has) to Allen head bolts, also the earlier Cinelli SC cast BB shell (AKA the "can-opener" with those stay sockets).
A surprise is the lack of shifter bosses, but could be it's been built to a customer's spec.
Would not be surprised given the size of frame if this used a heavier gauge 531 tube for at least the seat tube, which would account for the smaller (27.0 or 26.8) post. My bet is on 27.0