Now how many Columbus SL frames have you seen that take a 26.4 post, especially Italian one's of this era? Which begs the question, does this frame really have a Columbus SL seat tube?
I'm guessing that your frame had a SL down tube in the seat tube position. Down tubes were butted at both ends, and if used as a seat tube, would require a smaller diameter seat post to fit the butted end. Which begs the question, if the DT was used in the ST position, is the single-butted ST in the DT position?
Kevin Ko
Eugene, OR