That is a Federal two piece buckle....that being 1/2 of the buckle. General Service Buckle showing the Eagle with a shield covered by the stripes of the U.S. Flag(Union)..... I have some of the General Service Eagle buttons worn by northern soldiers. Got them off the battlefields here in Virginia....They match your buckle.....This design was used before the Civil War as well as during it and after for a bit. Your piece would interlock into the center of another piece which would make up the whole buckle. Here is a picture of a similar one of what you have although the design is abit different and shows "U.S." instead of the Eagle. But both were used quite often.
P.S.--The one posted above is a fake