It appears to be a buckle ...probably a sash-buckle due to its thin-ness, but there are a few Military ones which are made of thin stamped-brass. Note the small "tongue-hook" on the back of yours and on the back of the sash-buckle posted here in the What-Is-It? forum last week: http://www.treasurenet.com/forums/w...r-holes-rensselaer-county-ny-help-buckle.html
We need to see additional close-up photos after you clean all the dirt off of it.