It's believed that the holed stones may have acted as gateways to the entrances of cairns. Originally there were probably blocking disks, of stone, that kept the places shut. I think there are sites in France that have both stones. There is a suitable blocking stone here - http://www.themodernantiquarian.com/post/20523 - but I'm not sure how it would measure up to the hole. I could make a template in cardboard, next month, I guess.