I have been to Reynolds' site and it defines several types of tubing, including 525, but a fair number of bikes have 520. What exactly is 520? The ads for 520 bikes talk about it being butted. waht is the extent of butting on 520?
520 is a 4130 chromoly steel, with double butted main tubes and (possibly) plain guage stays. It is found on budget tourers and quality hybrids, plus a few track and CX bikes generally in the sub $1000 range.
The butts are designed for TIG welding, so are a bit thicker and shorter than on 531.
Reynolds tubes are often available in different diamters, cross-section shapes and butting profiles, the number just refers to the material.
Orbit cycles reckon it's heat treated, but I think that is a misprint.