before the "T" powder there were the S, J and M, with the corresponding proof marks.
I have a Pirlet boxlock with all three stamps PS, PJ and PM.
The "S powder was deemed the hardest to pass and was used by la manu long after T was introduced.
The "R" only applies to Ideal model # as in "No 3R".
The poster is correct in that it was supposed to be used in conjunction with Nitro proof relative to BP proof. It stands for "renforce" e.g. reinforced.
Of course, this went away in the 20s, when nitro was the standard.