850 kg/cm2 proof was commonly used for entry level Basque 12/70 guns of the 50's through the early 70's. Better grades carried 900 or 1000 kg/cm2 proof with some 12/70 pigeon guns 1200 kg/cm2.
Service pressure for 900 kg/cm2 would be about 9500 psi
IF the barrel is 18.3 = .720" and the L barrel at 17.1 = .673" that is serious choke!
I can't read the barrel weight but it's likely a game rather than Pigeon gun.

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