Page 20 of the published UK book: "RULES OF PROOF 1954" contains the following chart titled: "APPENDIX II, SCALE I, FOR THE PROOF OF BARRELS AND ARMS OF THE FIRST CLASS"
From this chart provisional black powder proof of 20 gauge 2 1/2 and 2 3/4 inch cartridges using TP black powder ( TP = Tower Proof powder) was 205 grains with 7/8 oz. number 6 shot.
From this same chart black powder Definitive Proof of 20 gauge 2 1/2 inch cartridge proof was 136 grains of TSP (Tower Special Proof black powder) and 1 5/16 oz. number 6 shot. Additionally the black powder Definitive Proof load for 20 gauge 2 3/4 inch cartridge was 157 grains of TSP with 1 1/2 oz. number 6 shot.
The Definitive Proof loadings of Smokeless Powder is not listed with the chart reading: YET TO BE DETERMINED.
Those loads should quickly weed out the weak from the strong.
I suspect that these black powder proof loads had been used for many previous decades.
Yes, "2-piper" they are extremely heavy to my mind also.
