Tim,
The telltale evidence of a replaced butt plug is the rim of the horn around the plug. It has an uneven edge with multiple small knife-cut marks, all uneven, which "cleaned up" the shortened end. Also note the unevenness of the current plug nail locations, some back a short distance in the horn body and a few right up on the edge of the horn body. Those details do not match the overall quality of the horn itself, strongly suggesting a later hand was involved in the horn, i.e. replacing its plug. The plug itself, with the many small knife cuts, i.e. "flats," covering its surface, also suggests a replaced plug when compared to the quality of the horn body itself. Lastly, from the probable time/location this horn was made, the high domed plug is not historically correct or in keeping with the rest of the horn body. I fully agree the plug has been in the horn a long time and its patina looks great, but I strongly doubt it was the original plug in the horn.
Shelby Gallien