Trev, that's definitely the "Anson" model, exactly like mine, and less than 150 numbers apart. Proofs . . . those are exactly what you would expect to find for a British gun made in Birmingham between 1925-54. The 12 over a C in a diamond simply indicates it's a 12 gauge and the chambers are shorter than 3". The "choke" means that the barrel has at least a minimum amount of choke; going from recall here, but I think it means more than .006 or something like that.
Using a Galazan chamber gauge, the chambers on my A&N's measure about as dead on at 2 1/2" as you're going to get.
Powder charge . . . Unfortunately, all you can tell from the proofs is that at the time the gun was built, it could be used safely with any 2 1/2" British shell containing 1 1/8 oz shot. I have been reloading standard 2 3/4" hulls to appropriate low pressures and shooting them in my guns--a lot--for the past 3 years. Mostly 7/8 oz target loads, but some 1 oz and 1 1/8 oz hunting loads. No problems whatsoever.
