Old Ned":3kpvztik said:Dave Marsh (who is one of the top experts on Raleigh SB frames having worked there) told me that 753 was 26.8 and 531 was 27.2.
You could ring him at his shop and ask his opinion.
I think it's only early (metric) 753 that was 26.8.
I've got a mid-80s 753 frame and it takes a 27.2 seatpin. Tony Oliver in his book said 753, if built "properly" should be 27.4.