Mike, re dating your Harkom, can you give us a clear pic of the proof marks on the underside of the barrels, or PM me with such pics? (you can send a much larger and clearer pic by PM, if need be.) The engraving pattern on locks, etc., and long tang on your buttplate says older rather than younger. Proof marks may at least let us date to pre 1853 or so, or after.
Aloha, Ka'imiloa