I thought I would update this thread for the kind folks that had asked questions regarding some of the markings on the powder horn, and to provide the hand-carved measure's capacity. These items are normally kept at our family cabin, a long drive away, though I recently had the opportunity to go there and retrieve them for further scrutiny.
Firstly, I was curious what the carved powder measure's capacity actually was; my thinking was if the measure did not correspond to a useful shotgun load, then it likely was not contemporary to the powder horn and likely made much later, as someone's bad guess.
Not wanting to waste/contaminate good (and scarce!) blackpowder, I used some old "P" grade Pyrodex I had on hand; a little finer that what probably would be used, but close enough. I found the measure to hold right about 75 grains, or 2-3/4 drams of powder. If used to measure bird shot too, this would be about 1-1/8 oz. Assuming this was for a 12 ga. (it fits nicely inside the tube of a 12), this would closely match what I found on Goex's website for a light ML shotgun load (76 gr for 1-1/8 oz shot). So, this measure may very well have been in contemporary use with the powder horn.
Lastly, some readers were speculating as to the origins of the stains or markings on the horn, so I took some close-ups photos of the horn's top and bottom that I hope can resolve the questions.
Horn Top:
Horn Bottom:
The horn top shows at least three spots, with stains running down to the horn's bottom. These appear to me to be drips from stained water or the like, and not shadows left from anything that had been attached to it. But maybe someone else will see something I'm overlooking!
The top photo also shows several dents in the horn's top (I count 6 or 7), right about where the sixth tack would have been. This would be right against the shooter's body when worn. Since there isn't much discoloration, it looks like the tack was maybe knocked off a long time ago, maybe the result of whatever caused those dents.
Also, the walnut plug shows a wear line; it seems the horn was attached by a fine string when worn by the user. When I first saw it, it had a short leather thong attached there, much thicker than the wear line.
Well, I think I've beat this poor horse to death - I hope this was of some interest and thanks again for all your insights!