I thought I remembered seeing the 531C gauge as being the heavier gauge tubing for larger riders and/or stiffer frames. I have one in 531C and it seems stiffer than just about any 531 in a similar sized bike.
Garth Libre in Miami Fl USA - (The author makes no guarantee as to accuracy of any claims real or implied. YMMV substantially and stiffness is largely in the mind of the stiffee.)