Could this be an artifact from WWII? Estimated Japanese military and civilian deaths exceeded 270,000. There are caves all over the island with human remains. The caves are supposed to off-limits but people still explore them at their own peril. You might compare the picture with the skulls at my site. We have human and primate skull replicas at http://www.skullarama.com. These are affordable museum-quality products for display or educational purposes.