So in summary

In late 1935, the Proof House established that 2" proof and service pressures were the same as 2 1/2" - 3 Dr. Eq. with 1 1/8 oz. shot with a mean pressure of 3 1/4 tons by LUP = 9,800 psi by Burrard’s conversion.

Henry Sharp in Shooting Times, April 1, 1939 re: 2" loads
This latest issue with its load of 28 grains of Empire powder and 1 oz. of shot will be obtainable for the coming season as an alternative to the cartridge with the original loading of 26 grains of Smokeless Diamond and 7/8 oz. of shot.
The same loads are listed in the 1948 Edition of Burrard; with no 36 gr., 42 gr., nor Dense Smokeless listed
Curtis’s & Harvey “Diamond Smokeless” (introduced in 1903) and Nobel’s Explosive Co.'s “Empire” were both 11 grains/dram equivalent
26 grains = 2.36 Dr. Eq (roughly the standard (British) 20g 7/8 oz. 2 1/3 Dr. Eq. load)
28 grains = 2.55 Dr. Eq. (roughly the standard 16g 1 oz. 2 1/2 Dr. Eq load)

Burrard's The Modern Shotgun, Vol. II "The Cartridge", 1955 3rd Edition p. 157 lists standard pressures for cartridges
12g 2 1/2" 1 1/16 oz. 3 Dr. Eq. Service 2.7 tons = 7,952 psi; Max. 3.5 tons = 10,640 psi
12g 2" 7/8 oz. 2.36 Dr. Eq. (26 grains of 33 grain = 3 Dr.Eq. powder) Service 2.25 tons = 6,440 psi; Max. 3.0 = 8,960 psi

Under the 1954 Rules of Proof 2” chamber guns were proved for 2 3/4 tons = 8,193 psi max. service load.

WHAT WE DON'T KNOW
We've seen 2" guns marked 3/4, 7/8, 1 and 1 1/8 oz proof
Were the proof and service pressures for 3/4 oz. and 7/8 oz lower than 1 and 1 1/8 oz??

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