Just ran on this thread today, so a late post here.
Without examining the horn in hand, I am almost certain that the engraving was done by Richard Cresey (1737?-1809) as it has all of his "key features" in both the British Coat of Arms and the $#*! Horse. (The Honourable Company of Horners, The Monograph Series, Volume 1, The French & Indian and Revolutionary Wars Engraving Of Richard Cresey 'The Hellhorse Powder Horn Engraver'.)
Interesting piece. If it indeed has a divider, I would concur that it a "salt and pepper horn" or spice horn.
It is on my bucket list to get to Boston to see and handle it in person.
John