Thanks! I got this from the link you gave:
"We had a discussion on this at the ASME Section IX Subcommittee meeting a year or two ago. Dr. D'or Doty, who probably was the longest subcommittee member at that time said that the "P" originally designated materials approved for welded "pressure" applications. The "S" numbers were developed sometime after P-Numbers and originally referred to materials approved by Code Cases for "structural" applications. That description would no longer be correct today, as there are many S-number materials that are used for pressure applications, such as piping."
This solves the "P" in P-no. and probably the "S" in SA-xxx. Some puzzle still remain unsolved. I did not go through all the threads yet so Im not sure if I'll find all the answer there. would I?
Happy new year!