just to ad to the confusion, 26.8 mm came up when googling your question.
are you not able to get hold of a vernier callper and measure?
.... but just noticed your post states it hs been fitted with the 27.2 - if someone has forced an oversize post into the seat tube, it does not bode well for the seat tube. how tight is tight? the tube may be damaged so the correct sized post, be it 27.0 or 26.8 will now be too loose.