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