José is probably fairly accurate to call it a "hook", but that work might give you the wrong impression. Basically what was in the insert area was another piece of wood with a slant cut at the bottom to help keep the sheath secured when stuck into a sash or waistband.
I took a couple of quick phone pics of my example. Mine covers the entire width of the sheath and was carved directly from the main piece of the sheath whereas the one on yours would be a separate piece and thinner, fitting into that insert space, either adhered with damar or wrapped with rattan or both.
Soory for the bad photos, but you should be able to get the idea from them.
