Thank you Stephen
In summary

1925 12g proof and service loads as published by Burrard
No 2" listed (and as noted proved to the same as 2 1/2")
2 1/2" and 2 5/8" Definitive Proof 178 grains / 6 1/2 Drams T.S.P. with 1 11/16 oz. Service 3 Drams with 1 1/8 oz.

Burrard "The Modern Shotgun" Vol. III, 2nd Edition
p. 217
"The 2-inch 12-bore...was originally introduced as an alternative to the 20-bore and fired the standard 2 1/2-inch 20-bore shot charge of 3/4 ounce. Experience proved that this shot charge was too light. So the shot charge was increased to the standard 2 1/2-inch 16-bore weight of 7/8 ounce."

c. 1935 the Proof House determined that 2" chambers would be proved the same as 2 1/2" (See back on p. 5 courtesy of Vic)

2" post-1954 Definitive Proof 170 grains T.S.P. and 1-5/8 oz.

Burrard published in 1955
12g 2 1/2” 1 1/16 oz. 3 Dr. Eq. Standard Service 7,952 psi; Max. Service 10,640 psi
12g 2” 7/8 oz. 2.36 Dr. Eq. Standard Service 6,440 psi; Max. Service 8,960 psi

We'll soon know, but it is likely, post-1954 2" were proved 2 3/4 TONS - Highest Mean Service Pressure 8,120 psi (by Burrard's conversion) = 560 BAR

John Brindle, author of Shotgun Shooting: Techniques & Technology published a review of Proof and Service pressures in Part 5 of his series in The Double Gun Journal, “Black Powder & Smokeless, Damascus & Steel”; Volume 5, Issue 3, 1994, “Some Modern Fallacies Part 5”, p. 11. His estimated post-1954 but pre-CIP standard pressures by LUP converted to piezo transducer PS
12g 2 1/2” Standard Service - 6,800 psi; Max. Service - 8,800 psi; Proof - 12,250 psi

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