It's in the Minutes of the Wagon Standards Subcommittee Paul, minute 5821 dated 28.2.63, and now I have it here it specifically says 'This code figure would supersede existing "XP" and "Star" markings. It goes on to list specific vehicle types/classes. It has a handwritten note, 'Amended - See Minute 5851', which I dont think I have any notes of, but as it's clearly only a proposal, that might refer to its non-adoption.
If you need anything more, get one of my friends to let you have my email addy (as you can't PM me), or if you find it it, would you report back on whether 5851 adds anything?