My great grandfather was Joseph Cullum Bishop (J.C.) from Floyd County, Virginia. I obtained a copy of this letter from the Library of Virginia. It was included with his application for disable veteran pension. His service record shows him Company G of the 17th Regiment, Va. infantry.
The letter that he wrote continued after the release from Castle Thunder "wen to Pickets Division Caurses Brigade under Robert E Lee. My captain name was Robert Knox of Richmond,Va." and was there until Lee surrendered.
So did these soldiers just move from division to another? I'm not very knowledgeable about military .