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@Momentum
There's a lot of info on the internet about the differences between tubing, but the different Reynolds numbers do not indicate "purity". They are all steel tubing and steel is an alloy. If you make it "purer" you will get iron and there's a reason iron bikes aren't popular.
531 originally stood for the percentages of the alloying elements - something like 5% chromium, 3% molybdenum (or whatever) and 1% something else. Or maybe it's 0.5%, 0.3%, 0.1% - you get the message.
For a long time Reynolds made a variety of 531 tubesets (eg 531c, 531ST) for different applications. They varied tube sizes, butting etc. Then they came up with new alloys and made 653 and 753 which had different properties (ie stronger) once heated. These aren't around now but you can find some really nice frames made from them. My first fixed gear was a converted 653 frame - lovely bike until a cabbie rearranged its geometry for me.
Now most of their tubesets (631, 725, 853 and 953) have this "heat-hardening" stuff. The main difference as you go up the range is that the tubesets are lighter cos they can be made thinner or they are stiffer at the same weight. 953 is the newest - it's almost stainless and very light so it's pretty cool. There's a big price tag for a 953 frame though.