It sounds like a cape gun, but not sure that is the correct term. Here are the Belgian proof and trade marks.http://damascus-barrels.com/bp.html
I would be great if you could post a picture or two.
AJ Plate was a California firearms merchant and retailer. He was active during the "Gold Rush", circa 1850's. He is best known for a loosing a lawsuit to Henry Deringer of Philadelphia. I have seen Plate marked deringer's bring $1000-$2000 at auction.
There is an AJ Plate shotgun on display at the NRA Museum.
Depending on condition, you may have a very unique and valuable gun.