Yes, musket butt plate. But I'm thinking probably not Brown Bess because of the engraving. It's a military gun, hence odds are against it being engraved. Some might say officers gun, but generally officers didn't carry the same arms as the infantry grunts. Non-coms carried spontoon's. My swag is it's civilian musket, owned by someone that could afford a higher class, fancier weapon. Lots and lots of muskets in those days with the same style of butt plates as a Bess. My swag also is it's not a butt plate for an Indian trade musket, even a chiefs grade had a flat butt-plate that was nailed on. Here are three pictures of original Brown Bess butt plates.
<img src="http://www.treasurenet.com/forums/attachment.php?attachmentid=1050565"/><img src="http://www.treasurenet.com/forums/attachment.php?attachmentid=1050566"/>
<img src="http://www.treasurenet.com/forums/attachment.php?attachmentid=1050572"/>