To answer the original question as asked: No. There is no standard 531 seat tube size. 27.2, 27.0, 26.8, 26.6 (some 531 Professional metric) 26.4, 26.2 and - in one of Neal Lerner's Super Course cases, 26.0.
I will not try to explain why this is the case, but I think these sizes are sufficient proof that you cannot second-guess seatpost size in a 531 frame.
-Kurt
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