I personally think that it's very unlikely that the hilt is from human bone, the dagger is 31,4 cm long, so the handle will be a little bit under 10 cm long, which human bone would fit this size?
I think that the dagger could be a "coup de grace" dagger from a rich/important person from the mid. to end of the 19th century and the bone is maybe from a deer leg. I asked which bone it could be because the seller brought human bone in play but I personally think that this is most unlikely.

I also don't think that it is a Freemason dagger, but this is a personal guess since I just don't know enough about this category of daggers.
But it would be interesting to read what others think about this dagger!