A previous internet research project revealed that indeed 531c is the later form of 531, and is heavier, stiffer, for stronger heavier riders. The c stands for competition and it's funny that I continually run into prospective bikes which are of the c variety even though, I am never more than 160 lbs, and definitely not a big gear cruncher either. The team Raliegh for example was made in later versions in both 531C and 700 series. The 531C was once again for the heavier riders with more leg torque. Another bike I have is made with stiff Japanese tubing which is even stiffer than 531c. The Columbus SL seems more comfortable, more forgiving and yet still plenty stiff enough for a guy like me.
Garth Libre in Miami Fl.