you sure that's not 531? seems odd a japanese model would have 531. Seatpost diameter depends on weather it is double butted or straight gauge. Could be 27.2mm I think 26.8mm for straight gauge if it is 531.
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1 Super Record bike, 1 Nuovo Record bike, 1 Pista, 1 Road, 1 Cyclocross/Allrounder, 1 MTB, 1 Touring, 1 Fixed gear