The oil painting described here is a color rendering of Alonzo Chappel's work (seen here). Looked familiar, eh? I had seen the engraving before.
Here's the neat part. Chappel ran something like Andy Warhol's art factory. His assistants created engravings from oil painting originals. They were all based on Long Island. This painting was found on Long Island.
Is this Chappel's original painting from which the engravings followed? It is not known to the web compilers of his art. It is a find but is it Chappel's work or a rendering of one of his engravings by a student or third party? Answer that question and your Antiques Roadshow moment has arrived.
Chappel did many nice pieces. This Scott graced an Eisenhower book cover. Gather clues by comparing and contrasting the level of technique in Scott's face and horse with GBM's face and horse.
And have yourself a nice Civil War browse.