Found this this in the back of a 1860s house in a old bottle dump. It is about 18 by 7 mm. Probably copper. If anybody knows anything that would be great.
It is part of an early-1800s cufflink. Note that the loop is set quite deeply into the back. That's not proper for a button loop. Unfortunately I do not have a photo of an intact cufflink of that type. It consists of two small, narrow oval-shaped "buttons" connected by a "chain-link" of brass wire.