Originally Posted by
old and new
Thought so... I'd spotted this post some time ago. Search some too. I remember them changing from 631 to 4130 early 2000s. 631's good stuff, 4130 can be any chromoly, not bad I'm sure in the case of Surly but if I had a choice, I would choose 631. Your bike has an edge. 520 qualifies as chromoly and 631's a step above. Read Bob Jackson sites, there are two; an old one called "home" and the store one.
They've a good overview of Reynolds, more than an overview and it's in plain terms.
725 is Chromoly too, and a step above 631. It's just tough to say without knowing more. What IS known is that 631 is damn good stuff!