i first saw it in a book called "the meeting of the tracks.rock art in ancient wales". there's a full chapter on it.
there are 3 copies of the stone, 1 in cardiff museum, 1 in brecon museum, and 1 in llywel church(near trecastle,powys)
there are human type figures in the carvings,they look to me like they may be a later jokey addition,but who can say.