Old thread, but I got my hands on some 531 SL. Seems the TT and ST are 0.7-0.5, DT 0.8-0.5 and stays are 0.6. Pretty thin for 531 alloy. Assuming the HT, steerer and fork blades are the same, once cut a set of 531 SL tubes will be some 180g lighter than a std 531.