There is no indication the horn collar ever actually split into two pieces, or that glue was ever applied in any manner to the crack...but perhaps time might hide any evidence.
We may be over-thinking this repair. The ring inner diameter isn't far off from the outer diameter of the bead above it, so my guess is that, with a little lubricant and perhaps softening the horn, it wasn't a big deal to force it over the above ring. Further, I think the lower bead, now gone and turned into a slightly tapered cylinder, had the critical dimension at its base, i.e. slightly wider than the iron ring, to seat the ring tightly and force the cracking sides together. If you look closely at the top of the iron ring, there are very slight gaps in some places, suggesting the horn surface below it was tapered, seating tightly at the bottom, but not so tightly at the top.
As to where I got it, if I said, it would taint the discussion about where it might have been made...and I'm not sure it was made in the state where the antique dealer sold it from...and he had no knowledge of where it originated from. I'd like to get unbiased opinions as to the most probable area/areas this horn may have come from. But then, I collect primarily out of the Midwest, and look harder for Kentucky items than any others. That could be a hint, or maybe not!
Tim, I admire your tenacity and willingness to experiment to determine how this repair was made. You have provided a learning experience for me and others that's been enjoyable. Nice to find a neat horn once in a while that generates an interest in others. Shelby Gallien