Old Asian bone carving/girl with compartment through head revealing miniscule animal carvings on seeds.

This girl is very old. It was in my late husband's collection of many items. I've always wondered about her..her origin, her timeline, etc. What is unusual to me is that the top of her head at her hairline comes off to reveal a secret interior and in that interior are the teeny, tiny sculptured animals. I have no idea if they were meant to be part of the secret or were put there later. There aren't as many animals as there there used to be as it was something to show everyone at one time and we continued to pour them out. I've tried to research to no avail. I assume ox bone?? Just can't find anything similar. I am very curious as to origin and possibly how old. Trust me, it's been around here for more than 1/2 of a century, I have no idea before then.