There appears to be some rust on the broken-off stud on the brass disc's back. Harness decorations usually don't have an IRON stud in the back for attachment. Therefore, I agree with Carolina Tom, it is a furniture decoration, such as from an upholstered sofa or chair, or a trunk's "reinforcing-bands" (as seen in the photos below). Furniture decoration discs tend to have a single iron spike in the back for attaching to the wood underbody of the upholstered furniture or trunk under the bands.