I visited this stone last summer. Very easy to access as it is on the verge of the road BUT the road is quite busy so parking is a problem. The broken small wooden fencing was still there - just. Not much to look at but easy to find.
From Brecon on A40 heading towards Abergavenny. The Stone stands on the verge of the road where the dual carriageway ends just before the village of Llanhamlach. It is protected by wooden fencing, now in a bad state of repair.
I have read that this stone may not be prehistoric, and I mention it here as it is less than 1km away from the Ty Illtyd burial chamber.
The stone is recorded on OS maps.
Square in plan and approx. 1m high. It looks promising on approach as the faces towards the road are eroded and weathered. However upon closer examination two of the edges appear to be neatly chamfered at 45 degrees. These chamfers also appear to show diagonal tooling marks.
I am assuming it is the remains of a preaching cross or boundary marker. It is well weathered and probably ancient, worth looking at and making up your own mind, if you are intending to visit the nearby burial chamber.
As Elderford hints, perhaps this stone is a bit young. The coflein record doesn't commit itself to any period but does admit the stone is on the line of an allegedly Roman road. Still, the romans had to put their roads somewhere. Marie Trevelyan calls the stone 'Maen yr Ast', contracted to 'Mannest' - or 'The Bitch Stone', presumably alluding, like a number of other names, to greyhounds (and perhaps Ceridwen taking the form of a greyhound?)
On Coflein its alternative name is the Peterstone; Peterstone Court lies across the road.
(M. Trevelyan, 'Folk lore and Folk Stories of Wales', 1909)