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.
Last edited by MichaelW; 05-29-03 at 11:33 AM.