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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?

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The original patents were 14 year patents and would have expired in 1872. They had hearing with the patent commissioner for extensions in 1872 which would have extended the patent for an additional 7 years to 1879. It looks like most of these extension were granted by the patent office and not congress at this time. Both of these patents were also re-issued in 1869 but that would not have changed the expiration date of the patent.

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