As far as I can see this stone is not listed anywhere, so it may not be prehistoric. Having said that it certainly looks the part, but it’s so extremely obvious from the road that I can’t believe it might have been overlooked by the archaeologists.
I’ve named the stone Ffridd y Beudail after the nearby farm house. I’m afraid the grid reference is not very accurate, because I didn’t have my GPS with me when I visited.