1896 Rules
The pressures were determined by lead crushers (LUP) so modern transducer numbers would be
10 - 14% higher
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA2&dqPowders used for proof are given as T.P. (Tower Proof) “which shall be of strength equal to Waltham Abbey “R.F.G. 2” and of grain varying between Nos. 4 and 5, that known as Curtis and Harvey's “T.S.” (Treble Strong) No.2 [for Supplementary Nitro Proof with 4 1/2 Drams and 1 2/3 oz. “soft” No. 6 shot] and, that known as “Col. Hawkers Duck Gun Powder”, and the nitro-powder known as Cordite, or any other description of nitro-powder which may hereafter, from time to time, be adopted by His Majesty's War Dept.”
Curtis & Harvey “T.S.” (Treble Strong) No. 6 (84 grain = 3 Dr. Eq.) was coarse Black Powder somewhat similar (but not equivalent) to Fg.
“T.S.” was developed in 1871 for the .577/450 Boxer-Henry cartridge used in the Martini-Henry rifle. It was a precursor to “R.F.G.2” (Rifled Fine Grain 2) manufactured at the Royal Gunpowder Mills, Essex, adopted in 1873.
C&H, “T.S.” No. 4 (82 gr. = 3 Dr. Eq.) medium grain similar to FFg
C&H, “T.S.” No. 2 (72 gr. = 3 Dr. Eq.) fine grain similar to FFFg.
I don't have the Black Powder Rules after 1925, but suspect they were similar
The 2 1/2” & 2 5/8” 12g Definitive Proof remained 6 1/2 Drams BP with 1 2/3 oz. shot
The 2 3/4” 12g Definitive Proof was 7 1/2 Drams BP with 2 oz. shot.