Full size close up in focus images of every mark on the barrel and action flats would help.

The lettre annale would document the exact DOM

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I'm sorry but I've never heard of "Blake" powder, and could not find it on an internet search.

Does it carry Smokeless Powder proof?

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Is the original Certificat d’epreuve no longer with the gun?

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After the proof house revisions of 1924:
“The pressure developed, measured by crusher type device, lower or equal to 600 kg per square centimeter for sizes 16, l2, 10, 8 & 4; 670 kg per square inch for sizes 20, 24 and smaller.”
600 kg/cm2 = 8534 psi maximum SERVICE pressure;
670 kg/cm2 = 9530 psi maximum SERVICE pressure
+ 10 - 14% by piezoelectric transducer measurement

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