The pistol is actually a Model of 1900 made by Fabrique Nationale (FN) and designed by Browning. These were used extensively during WWI by trench sweepers and Belgian/French officers. Modern repro magazines for them are available from gun parts sources (try numrich online). Good luck. FYI, Brevete means "patent" and SGDG stands for "Sans Guarantee Du Government," which means without the guarantee of the government. The name of the gun is FN Model 1900 or Browning "Old Model."
If you have the FN 1900, also called the Browning #1, it was produced from 1899-1907 and there were 700,000 made. It will have "Fabrique Nationale-Herstal-Belgique (Brownings Patent)" inascribed on the left side of the pistol. Under that it will have Fabrique Nationale's icon with a small picture of a pistol and a circle around it. Under that it will have " BREVETE-S.G.D.G." which is a french term for the patent process. It was used on many things including clocks. Hope this helps.
Up to 500 USD
Sounds like a Browning Model 1900. SGDG is means "Patented, without guarantee of the government" Try this website: http://www.gunsworld.com/world/browng1910_us.html
You will have to call Browning to find out.
There was a Browning 12 gauge with the patent 2203378-223386, that was sold for $999.00 at an online auction.
Go to the Browning website, look under Customer Service. You will need the serial number, not a patent number.
OK, what is your question?
Stevens 520 pump
I would have to assume that you have a semi-auto shotgun.If your shotgun has a slight hump to the back of the reciever then it was a shotgun made with a browning patent and would have to be listed on the gun along with the royaltys paid to his patent.
Can't be answered without a detailed description of the weapon.
You will have to call Browning to find out.
Contact Browning or compare against known examples.