Well it does look about the right size for an inkwell, but having 5 holes is quite unusual. Most (old) inkwells only have one ? But then I'm no expert.
If you do go back for a look at the area, check out the local rocks and see if it may have been fashioned from local material, or bought in from somewhere else. If its local rock, it suggest it was made by someone living in the area. .... In fact if you can identify the type of rock it is made from it may suggest where it came from. I'll guess that it was probably made by someone locally, I don't think it is the sort of thing which would have been carried a long way (but please prove me wrong).