Your brass lock sounds like one manufactured by E.F.Fraim Lock Company in Lancaster, PA, which was in business from 1881 to 1955. Although it has "US" on both sides, it has nothing to do with the Civil War or the U.S. Army. The lock is listed in "The Padlock Collector" by Franklin M. Arnall in the "warded' section. The mechanism is pretty simple to open (a basic warded padlock) so it's unlikely that it was used for anything valuable (like a mail or gold shipment). We have found several on late nineteenth century farmsteads in northern New Jersey. Not a very romantic story... John Sapanara, Pascack Historical Society, Bergen County, NJ
These show up on e-bay occasionally as a "cavalry" or "Civil war" padlock.