James Purdey Best SLE made in 1907, barrels 30", chambers 3" (12/LC), nitro proof 1 1/4 oz, bores 12/1 (for brass cases).
And weight is 3,620 Kg or 7,98 lb - one pound heavier, than it has to be IMO.
Or the weight is OK?

It has 3" chambers. Original proof was for a heavy load. Hardly surprising it was built as a heavy gun. A standard game gun of the time would have 2 1/" chambers and weigh around 6lb 12oz

The reason for 3" chambers was Black Powder and 1 1/4 oz (35 gramms) of shot I guess. It is not heavy load, actually its standard shot weight for 12G. I would expect to see 1 1/2 oz nitro proof on gun with 3" chambers, that weights allmost 8 pounds.
I'm affraid there were some tricks with wood and lead weights in stock.

Heavy proof Purdeys are coveted by shooters in the US. The odd proof and chambering for brass shells would seem to back up Geno's statement. Geno, are the chambers square cornered or do they taper into a forcing cone? A Purdey letter would be interesting if it addresses the proving and chamber details.

How are we determining it was chambered for brass cases and if it is what difference could it make ?

I've a Purdey made in 1900. 7 pounds 8 ounces, 2 3/4" chambers, 30" barrels, "Modified & Full" (Really; more like full and rifle)side clips, self opener. Pigeon rib. Safety.
I shoot it better than anything I've ever had in my hands, and cherish is a word for how I feel about it. I am honored to own it.
It's not a light 2 1/2" game gun; but shells are so easily reloadable and available at 2 3/4" that I'm a happy camper.
It shoots 7/8 ounce loads like a dream.
Sam Ogle, Lincoln, NE

Geno:
If you own the gun, then why not invest in Purdey's research and find out exactly how, and for whom, the gun was originally built? Unless he's an immortal, the original owner is most assuredly dead.

Best, Kensal

Its not my gun, but the gun is for sale. I have Purdey Best made in 1909, pegion gun with 28" barrels chambered 2 1/2", nitro proof 1 1/8 oz, upper bite, side clips and this is light gun 7 lb 1 oz.
When I tried to find out who was the first owner of this gun I got message from Purdey&Son gun been sold to Russian gun dealer in St.Petersburg. I'm sure the same was with this 'heavy' gun. But there are two letters P.B. on gold oval and on top of leather case and there is just a little chance this gun belonged to Pavel Bure - the best Russian maker of fine watches in St.Pete.

Joe, bores marked 12/1 means backboring for thick wall brass cases and bores marked 11 means thin wall brass cases.

Bill, I can't check cones, the gun location is another city, but I have to ask the seller and to send request to Purdey. But there is one more problem, the seller asks too much IMO, $60k.
Take a look some photos
http://talks.guns.ru/forums/icons/forum_pictures/003628/thm/3628378.jpg
http://talks.guns.ru/forums/icons/forum_pictures/003628/thm/3628450.jpg
http://talks.guns.ru/forums/icons/forum_pictures/003628/thm/3628485.jpg
http://talks.guns.ru/forums/icons/forum_pictures/003628/thm/3628575.jpg