Most likely an un-issued belt and buckle.
This Civil War Federal pattern buff leather belt with brass keeper, and the original M1839 large oval “US” belt plate are probably in un-issued condition. There are no makers marks. The buff leather retains its dark original color, and traces of original luster cling to the protected areas of the belt plate.
Belt appears to be set up currently for a 36″ waist.
There is some light soiling on the inside of the unfinished buff leather and may have been used in display or having been in poor storage, the belt just shows no sign of any constant use from the period.