Part of the reason for the varied answers is that Reynolds offered the 531 butted seat tube in different gauges. One took a 27.2mm post, while the another took a 27.0mm post. There was even a very heavy gauge seat tube intended primarily for women's frames that used a 26.4mm post. Beyond that, you could get all kinds of variations depending on build-up, distortion and how agressively the tube was reamed/honed.