That's a "Model 1809" Prussian "Potsdam" musket. Although it's referred to as the Model 1809, they were manufacture in percussion (like you have) in the 1830s and maybe later. They were imported in large numbers early on in the U.S. Civil War. I am, however, not quite sure what that square dug-out area is on the cheek. They were designed with a scooped-out area there but to the best of my knowledge, not with a gouged-out square like that.
