Gas pipe tubes have thicker walls, therefore smaller internal diameter, therefore smaller diameter seatposts. If the seatpost diameter is over 26.4mm it's probably Reynolds something. If it's 27.2 then it's most likely to be butted 531.
@lae started
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Gas pipe tubes have thicker walls, therefore smaller internal diameter, therefore smaller diameter seatposts. If the seatpost diameter is over 26.4mm it's probably Reynolds something. If it's 27.2 then it's most likely to be butted 531.