I'll take a stab at this, even though I'm sure more informed persons will correct my errors.
The numbers in Reynolds tubing refer to the proportions of alloying elements. Manganese, Molybdenum and Chromium, if I recall correctly. Reynolds differed from Columbus in having a higher percentage of Manganese than Chromium, which supposedly gave the tube better resistance to heat damage.
531 is the oldest, then 753, then some other numbers, and then 853 and 653. 853 is a stronger alloy, and is heat treated and drawn in thinner tubes than 531 for a lighter weight tube set. 653 is supposed to be similar to 853, but lacks the heat treating. So 653 is lighter than 531 and a bit stronger, but not as strong/light as 853. But it is cheaper than 853.
I wouldn't suppose that there have been tremendous strides in steel metallurgy since 531 was introduced. Various iron alloys have been the subject of experimentation for centuries, and chrome-moly and manganese alloys have been around for a long, long time. The big technical innovation that lifted Reynolds to the top of the heap was butted tubing, invented by Alfred Milward Reynolds in 1887. This was originally used with Reynolds HM tubing, that had a higher manganese and lower moly content. The 531 formulation was introduced in 1935, and combined with the butting process made Reynolds top dog in bike tubing. (The Custom Bicycle, Kolin & de la Rosa, Rodale Press, 1979). It seems like 753 and 853 have been refinements, not wholesale changes.
531 is fairly easy to fabricate, allows cold setting, is reasonably light and makes a very tough but comfortable frame. It would be ideal for a lot of bikes if you want lugs. I'm not sure if 531 is suitable for Tig welding, so that may account for it's decline.
Let the qualifications and corrections commence.
Tom Adams, Kansas City
I just purchased that Bob Jackson frame from ebay. I looked up Reynolds tubing types on their web site and could find no reference to 653 tubing which this bike is. Is there a comprehensive site for tubing types somewhere or does anyone know how 653 fits into the Reynolds line up. A secondary question. I see a lot of emphasis on 531 tubing almost like it is the Holy Grail of bike tubing. Yet I see that is has been around since 1935 and surely metallurgy has progressed since then so why the fascination with this tubing. Since I am still new to the lightweight world this may seem like a stupid question but that never stopped me before.