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@Scilly.Suffolk
I'm happy to be corrected (knowing little about Sun), but I think 26.4mm is more likely: double-butted 531.
If you swing by your LBS they can run a reamer down the tube to clean it out: there'll be all manner of ancient grease down there, which won't help with getting an accurate measurement.
Like as not they'll have some seatposts knocking about, to double-check.