Etex is correct, it is a RevWar British bayonet scabbard frog hook. I remember it from a long-ago relic ID question here in the What-Is-It forum. See the photo below, posted back then by T-Net memberFMarion2ndRegSC.
I mean the following as a compliment for a good guess, not as an insult. Anduril made a good logical guess... but at only 2 inches long and 1/2-inch wide, this object is too small and weak to function as a rope cleat or even a cord cleat. It's difficult to accurately guess the ID of an object with nothing else in the photo for size-comparison. Anduril was right, it sure does look like a cleat... but it's too small for that.