I dug this today at a place that has produces colonial era items. Made of brass is 4 inches long and 1 1/2 wide at its wides point. I know it's vague but it's ornate.
It is definitely the broken-off top portion of a Colonial era flintlock musket's brass buttplate. It resembles a British army "Brown Bess" buttplate... but although I did some websearching for you I couldn't find an exact match. The photo of a Reproduction Brown Bess buttplate at the following website unfortunately doesn't show the buttplate's top, but it does show the distinctive "locking tab" which we see on the underside of your find. Military musket buttplates: Brown Bess Muskets, Charleville - Track of the Wolf
You nailed it cannonball guy. It's the top half of the L shape buttplate. Well I guess I better go find the other half. More reason to dig. I sent a giant link but it'd a pic of the whole thing.