No sleeved frame that I've seen ever took a 26.8 post (in the classic era - not counting "oversized" tubing) and I doubt any ever did. The sleeve required to step it down to 26.8 would be too thin. You can slip in a piece of shim stock to do this after the frame is brazed, but you couldn't reliably build such a thin sleeve into the frame. Cinellis with sleeves step all the way down to 26.2.
Someone asserted you'd have to use a plain gauge .049" seat tube to get 26.8 - not true - .049 (1.2 mm) seat tubes usually take 26.0 mm seatposts.
The normal wall thickness on a 28.6 mm (1-1/8") seat tube that takes 26.8 comfortably would be 0.8 mm. This is thinner than most any normal butt (ignoring ultralight tubes for the moment), refuting the theory that a single-butted seat tube was put in upside-down. Even an SL seat tube upside down would take a smaller post (26.6)
So most likely the tube is 0.8 at the top or it is 0.7 (Columbus SP) with a bit more distortion and/or less reaming than normal, preventing 27.0. The latter theory is most likely, IMO.
Mark Bulgier
Seattle WA USA