Here's a bike with 531p (professional) tubing. Its an 80's decal that says "531" with devilish red lattering rather than the traditional green lettering of "531c" tubing, and the red word "Professional" is squeezed in right below the "531" lettering.
The 531p tubing was a very light gauge of 531 tubing, a step above 531c. The 531p tubing has the same dimensions as 753, minus the tensile strength, so it probably builds up into a whippy frame, and may have been used mainly for the smaller sized-frames, or perhaps regular 531 (531c) tubes were substituted in some places (e.g. downtube) for larger frames.
Reynolds 531c / old531 is an 8/5/8 tubing Reynolds 531p and 753 are 7/4/7 tubing(s). Tange made a 6/3/6 tubing, i.e. for the SEKAI 5000.
At least you could cold set 531p tubing !!
This is oversimplifying things but I think it gets the point across.
- Don Gillies
San Diego, CA, USA