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It may be trickier, remember this is OLD, during the dates you mention there was no West Virginia yet as it was formed due to the Civil War, adding more to the trickiness is that a few years after that, Monongalia County and Harrison County both gave up some land to create Marion County. I have nothing to confirm that they meant Frederick, VA, but I've run across similar errors while attempting to do some research myself. It is of course possible, likely even, that since the dates you mention are pre-revolutionary war, back then the 13 colonies ran from the coast to the Mississippi River, that the county if it even existed back then, was large enough to encompass the area as well.