I came across a strange, circular stone with a central hole - located just below the rock piles/quarried rock, near the ancient cairns at Mynydd Llangatwg (Twr Pen-Cyrn) near Brynmawr. I have some good photos - but as a newbie, can't work out how to attach them...

The stone is lying flat and is around 1.5 metres in diameter and around 70 cms deep. It's not quite perfectly circular, it has some small irregularities at the edges, but does have a circular hole right through its centre. It is also pretty flat across the top. I wouldn't know exactly what to look for in terms of evidence of hand working, especially given its (apparent) age and weathering - but it certainly gives the sense of having been made or cut by humans.

The central hole is interesting too: it's very circular for 75% of its depth and then somewhat less regular towards the bottom.

Anyone else come across this specific stone? or others like it? Any ideas what it might be?