Thanks, that makes a good comparison!
Like said, at least by my kalasan handle I feel very confident that the hole is natural. When you look close you see the natural grain around the hole which is in the middle, I think to see the same by Ian's handle. Buffalo horn is hollow in the most of it's length and only at the end massive.
Yes, your's looks like the natural cavity. Notice the relatively smooth inner surface and the depth of the hole (compared to the surface of the opening).
However, what throws me with Ian's example is the considerable area of the opening and the relatively shallow depression: Considering the rather limited diameter of the pommel, one would expect the cavity to reach the front of the pommel or at least very close! (There is a slight angle to the grain - it might be possible that the tip of the cavity extends into the gripping area.)
Ian, can you ascertain whether the bottom of the hole is only resin/fillings or is there also solid horn? An X-ray would be nice to have...
I think it's a inadvertent accident from the carver. The hole in the handle from Ian's knive look very similar to my eyes. And the knife isn't fancy at all so I doubt that the hole is extra carved for a jimat.
True, maybe the resin was just supposed to patch the hole.
BTW, I don't think this really is an accident or missing skill of the carver - most likely these are lesser examples made to a budget while the largest horns or best pieces were reserved for VIPs...