Tim:
I was going through all the pictures I have of artillery or gunners horns, and couldn't find any with a map on it. There are some engraved artillery horns of course, but I couldn't find one with a map on it. Still, my nagging memory suggests there may be one in the collection at Fort Ticonderoga. As to the horn in the picture, the turned walnut base plug and threaded handle/stopper look dead right. I'd like to see a picture of the spout. As to the engraving, I believe it is not contemporary to the horn. In other words, the engraving was added later and should be considered spurious. In fact, it reminds me of the hand of a faker who did other Boston horns. There are three of his horns in Grancsay's book: The Benjamin Corey (1775) horn, the Valentine Prentice (1777) horn, and the Thomas Frye (1756) horn. I would add the Israel Putnam (1747) horn in the collection at Ft. Ticonderoga to this list as well. Swayze argues in his book that the Frye horn's engraving is not contemporary to the horn (p. 198). I agree.
Now, back to the horn in your picture. The outline of the Boston peninsula, the mill pond, the docks, and the hills on the horn all remind me of the map on the Corey horn (see the sketch in the back of Grancsay's book). Also, most Boston horns use more sweeping lines, less detailed shorelines. I haven't ever see a shoreline on an 18th C. horn outlined in the manner that this one has been done. All these details probably came from a 19th C. map. I believe this horn may have been an authentic gunners horn (I'd still like to see the spout), but I believe the engraving is spurious. Lee