Wow! What a beautiful piece! Sorry, no expert on regimental marks, but had to comment on this horn. I would agree with the period of late-18th or slightly earlier. Did you acquire this in the U.S? Europe? The old iron butt plate points to a continental origin (Despite the use of tin, iron, pewter, etc, in the colonies at that time, I have never seen a Rev War era Amer horn with anything else but wood bases). My impression is that Amer horns of the period were made of simple materials in a time when resources (metals) were scarce.
