That's a very nice horn. I'll defer to the experts regarding its age and any possible efforts toward restoration or enhancement. To my admittedly unprofessional eye, it looks like an old horn someone may have tried to clean up.
Vectoring off in a slightly different direction, I did a cursory Google search, using "Captain John Manuel 1781" as keywords. Interestingly, the first and only truly relevant hit was for a Captain John George Manuel (29 March 1893 - 10 June 1918), who was a Canadian First World War flying ace, officially credited with 13 victories. As the engraving on this powder horn clearly shows a Montreal/ St. Lawrence theme in association with the original owner's moniker, we wonder if there might be a family connection of some sort.
In any event, thanks for showing us this nice old horn. I think it's a beauty, regardless of its age.
Notchy Bob