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@Wheelnut
Okay, forget the commonly held belief that all 531 frames use a 27.2mm seatpin.
Reynolds 531, right from the 1930s, could be specified with different gauge tubes of plain gauge, single butted or double butted, depending on the frame-builder's preference and the size and purpose of the frame. Seat tubes were usually single butted even when the other two main tubes were double butted. Also, it was common practice to ream out the seat tube for a perfect fit because the tubes could distort slightly during the brazing process.
As such you can get 531 frames with seatpin diameters of 26.6, 28.8, 27.0 and 27.2mm. Also, a frame with, say, a 26.8mm seatpin does not imply that it is not as light as one with a 27.2mm seatpin.
Not sure if this clarifies or confuses matters...