The hilt with a recess for the throat of the scabbard, blade with US acceptance mark and S.K inspector's mark and grooved grip core should make this an Ames model 1840 mounted artillery saber type I. The recess in the guard for the scabbard dates this to 1860 or earlier. The earliest date should be 1845 although, according to Thillmann, a single sword with an 1844 date has been found.
The stuff on the knuckle bow looks like a unit marking, although not the way the Germanic states would mark them. We're always reading about how after the war the U.S. sold tons of small arms to European countries. Perhaps this sword was a mercenary during the Franco-PrussianWar.