Originally Posted by
juvela
We know someone has reduced the diameter of the seat post in order to fit it into this frame. My statement suggested possibility that they may have enlarged inside diameter of seat tube as well, in order to accomplish this fit...
While this is possible, it seems unlikely. Given the presence of a Cinelli BB shell, it is almost certainly a premium grade tubeset for the era and likely has not been reamed any more than to restore the tube to round and spec diameter. Metric tubing, in conjunction with a 26.6mm post, makes butted Reynolds 531 the most likely candidate.