Hi,
My first inclination is that it is from some sort of colonial made gun using a British export fowler buttplate such as the one below.
It also looks a bit like the buttplates used on some commercially contracted muskets used during the 18th century in North America. Normally, there would be two screws in the face of the plate and a lug and cross pin holding down the tang. In your case a top screw was used instead suggesting to me a gun made in America.
dave
