Sam Colt Signature Series 1848 Dragoon United States Marine Corp Third Generation Commemorative Nickel and Gold Plated Black Powder Cap and Ball .44 cal Revolver, I think.
Engravings should read "Sam Colt Semper Fidelis" on the backstrap, and "Address Sam Colt New York City" on the top of the receiver. Cylinder has an ornate cavalry scene, and reads "Colt's Patent" and "Model U.S.M.R." If you have the powder keg it should be ornate, and is engraved "Sam Colt" around the cap. Should have come in a case with a number of accoutrements. Hard to tell from the pictures though but I think that is what it is.