Hi Mr Hamhead. The reason I posted the stone on the Butterdon Hill site is that it is part of the row that starts on the hill and ends on Piles Hill where the stone stands. It is the second largest row on the moor.
The stone you photographed is called the longstone on the OS map and is part of the line of boundary stones that run across the southern moor from Western Beacon to Red Lake. The boundary stones also follow the line of the Butterdon row so there is every possibility it is an old stone reused . Hope this is helpfull.

Peace Lubin