Originally Posted by
Salamandrine
If there's no visible or detectable distortion or cracks, and it's 531, it's probably actually OK to ride. 531 is pretty ductile and tough. Even so I wouldn't buy it myself.
I don't believe Colnago used Reynolds tubing on any of their bikes, at least not any of the top end bikes.
It is likely some version of Columbus tubing, SL, SLX, or a variety of Gilco style tubing, depending on the frame.