William A. Clark had patents 20,192 (May 11, 1858), 21,597 (Sept, 28, 1858). He ran his factory from then to the civil war (1861 to 1865). After the war he restarted the factory and his bits are marked "PAT'D MAY 11, 1858 - Pat EXT'D".
Did he get a congressional extension and if so, when did the patent finally expire?