Thank you Rosinish: I have made some fresh research and came up with a candidate to fit the "L Powel" inscribed on the stone. My gt gt gt gt grandfather was the rev William Watkins, curate of Callwen. His daughter, Gwenllian Watkins, married Llewelyn Powel. Could he be the figure in relief? and could he have been a clegyman? His style and attire are similar to those of people like Howell/Hywel Harries of Trefeca.
Gadget: Yes, the Callwen I refer to is the church between Defynnog and the the top of the Swansea Valley - not far from Dan-yr-Ogof Caves.
I'm beginning to wonder whether this could be a boundary stone? or possibly a stone placed above the entrance to a house or other building?
By the way, if anyone is interested in our family history - families in Cwmwysg (Breconshire) and Llanddeusant (Carmarthenshire), let me know and I'll send a link to the family tree.