An unplated iron roller buckle or a longer than wider center bar harness types would be appropriate for the time and place.
Most buckles come plated but it's easy to remove with muriatic acid or heat or a combo of both - just don't breath the fumes. After wards you leave as is, age it, or apply faux Japanning or the blacksmith methods of dipping the heated buckle in linseed oil, wiping off the excess, and letting dry