This isn't a direct answer to your question, but I've worked on several knives with leather-washer handles (including Kabars) that had shrunk and gotten loose. In some cases I just replaced the entire handle by making new washers, but in other cases the owners wanted to keep the old handle, so what I did was make an aluminum spacer the same shape as one of the leather washers, left one end of the tang hole in the aluminum spacer open, and pushed/lightly hammered the aluminum spacer in between two of the leather washers to tighten the handle up. Basically, the finished aluminum spacer looks like a big "C".
The thickness of the aluminum used to make the spacer depends on how much of a gap needs to be filled between the leather washers to tighten the handle.