emike - Does that 20 pdr. have a lathe dimple? Was the sabot pre-rifled? I'm thinking it came from the same foundry as those seen on the first two pages below. I believe that to be Tredegar. None of their shells have lathe keys on the ogive and the sabot appears to be made of good quality rolled iron. It would help if I knew where it was found. Tredegar had stopped making 20 pounders by the end of 1862. Maybe a few in early 1863. The overall look of a Tredegar Parrott closely resembles the Federal Parrotts. I suspect Tredegar had received drawings and maybe patterns to work from directly from Robert Parrott who was a personal friend and fellow West Pointer. After the state of Virginia purchased 13 10 pdr. guns from New York in early 1861, Tredegar was contracted to make 5,000 rounds for the state militia. At that time, there was peace in the land. The third page below shows a 10 pdr dug by Mac Mason that looks like the typical Federal Parrott. It was not turned on a lathe and I agree with Mac that it represents the fine work Tredegar was capable of.
The fourth page below features your 20 pdr. with a Broun fuze. I have not seen that sabot on any other 20 pdr. from VA. Maybe I'll see some similar shells in the future to get a better idea of its origin. Right now I'm thinking "Navy Parrott" because they put so many impact fuzes in their shells. The Army shied away from them until the got Girardey's in early 1864.