Hiya Jude,
I dunno the answer to your question but they are very odd stones. I think the old chestnut about them being gateposts is a bit hard to buy as there is only one hole in each of the holed stones. None of the holes show any signs of uneven wear around the holes and none of the stones are paired to an opposite post.
I think there is some mileage in them being guide stones across a barren moor. But the purpose of the hole?????
What I love about them is their oddness.