Hi RobT.
I believe Ibrahiim is right on this being water buffalo horn. So it would seem the horn originated from somewhere the animal resides. Which does not narrow down the origin much LOL
The carving on the tip of the horn looks some what familiar. But I just can't place it. Seems like I've seen this style of carving on wood knife scabbards (?). Maybe another Forum member will reconize the carving style which may offer a better origin possibility.
It is a really neat looking horn I would not mind having in my collection. Great find, especially from a flea market. Maybe from the Philipines or Southeast Asia ?
As you mention, using the base of the horn for both fill and charging is less commonly seen, as on this example. However, the design goes back to the earliest periods. Below are a couple of pics of a European style horn utilizing the base of the horn for both functions. This is a contemporary made horn (sorry Moderator) copied from various original wood carvings of the Swiss/Landsknechts for a re-enactor. But it does illustrate the use of using the base of the horn only for both purposes.
Again, very nice and interesting horn you have. Hope another Forum member can better narrow down it's origin.
Rick