The closest on TMA might be this ogam stone which is near the East Millstone Cairn. This stone is just a few meters from the cairn.
http://www.themodernantiquarian.com/site/12188/millstone_hill_east.html
Note marks on the bottom of the stone!
http://i296.photobucket.com/albums/mm200/TJJackson66/048.jpg
The stone is in the ruined chapel of St Non next to St Non's healing well at St David's