653 is an interesting tubeset. The main tubes are 531 drawn to thinner walls than 531C, and the stays were initially 753, but later changed to 725. Several sources say the tubeset was developed by Reynolds as a result of Eddy Merckx criticizing frames made with all 753 as having a ride quality too harsh for some TdF and Giro d'Italia stages. Frames made from 653 resulted in an ultra stiff efficient frame while at the same time providing a forgiving ride when needed.