I haven't been on Butterdon Hill for many years and can't remember the stone. It does look quite old. I have looked in Jeremy Butlers books and can't see any mention of it there. There is mention of a triangulation stone near to the largest cairn on a very stoney area, could it be that ?
I will look through my other Dartmoor books over the weekend and if I find anything I will get back to you.
I have amended the comments with the two photos of the Butterdon Row saying that the stones are on Piles Hill to avoid any problems.
Peace Lubin