The bar is broken off of one end of your cast-brass version of buckle-cover (also called a buckle-shield), which was designed to protect the buckles on horse-harness straps from snagging on branches, underbrush, and overhanging vines, etc. Your version, was invented in 1879 by David Mosman of New Britain CT and patented by him in that year. Here's a photo of an intact one, which is stamped with the US Patent-date. These buckle-shields/covers are of course strictly post-civil-war era.