Another observation Rich. I was beginning to really focus on the horn itself and the relationship with the base plug. How did this horn get to this bad shape? Especially area around the base plug. Two things happen over time, wood swells and shrinks again and again with changes of the seasons. Likewise does a horn. Ever a 'blowing' horn sound different, or work well, on damp, rainy weather vs. dry times? Because of separation of plug to horn causing a few 'gaps' between horn and wood along with the horn delaminating in spots makes me think this was put together using more of a green horn and having hung near a heat source. Not seeing any visible cracking in the wood plug leads to me it was pretty seasoned to begin with and like Majorjoel noted, a tinkerer used a plug leftover from another source, etc. Over time, while maybe scraped smooth at the beginning, began to delaminate over time due to hanging near a heat source (fireplace, stove, etc) and took its' toll.
Of course, alot possibles as to why/how this horn came to be and during what time frame. Are we having fun yet?