I never had one but remember them well among Army helicopter crews in Vietnam, just as recounted by BudMan5.

I remember being told by some guys that the knives were often given to those who were still there. I was told they were prized because they could cut through the skin of the helicopters.

While SCUBA diving off the coast of Cam Ranh Bay I found a Bone knife. I carried it, but considering that I had no knife skills whatsoever, it was mere decoration. I still have it, but only to look at.