Ed,
Reynolds 531 was also available in a heavier tube just like the Columbus offerings at the time. Your Dawes is made from that tubeset.
Technically there is no absolute guarrentee that a bike that takes a 27.2 seat post is not SP or the heavier Reynolds. The reason is many companies ream the seat tube on SP frames to 27.2 to avoid having to stock two sizes of seatposts. Personally I object to that practice, but it's not the worst thing in the world.
Brian Baylis
La Mesa, CA