well very interesting to learn this is branded by them (Pearson) with diecut vinyl they produced: that's going to simplify your search for the provenance dramatically!
When you make contact, ask them what tubing was used: the 26.4 seat post (if correct size) would generally be one used with seamless DB tubing in a METRIC set, but AFA I can see that Cinelli BB shell does not have "French" in the casting (typically just below the the main rectangle with the Cinelli logo), so...it's mysterious.
A post that size with Imperial tubing would indicate a pretty heavy gauge, at least for the seat tube, or the builder put the tube in upside down and there's a butted section at the top. This has happened before, but it's a mistake.
I'd guess maybe the seat post is too small, but if so could be as much as almost a full mm (27.2 is the "usual" for DB 531 or Columbus SLseat tube), which you'd notice with a seriously pinched seat lug slot