This might not be prehistoric, and it's not mentioned by Coflein, but it is marked on the OS map so perhaps it should be findable for inspection.
A small unhewn and somewhat flat stone, so named on the Ordnance sheet, and said to be the capstone of a ruined cromlech. It is stated that about the year 1830 the stone rested upon several supports. The surrounding earth was excavated, and a quantity of coins and some other objects (said to have been of gold) were discovered. The coins, described as 'ffyrlingod,' 'farthings,' were dispersed in the neighbourhood, but recent inquiry has failed to trace any of them. The other articles are said to have been sent to Powis Castle.
The late Mr. David Roberts, Hendre Fawr, Llangynog, who has died (aged 90) since corroborating the above account, alluded to this monument as 'Bwrdd y Gwylliaid cochion,' 'the red outlaws' table.'
The supporting stones, if they ever existed, have been removed or broken up, and no trace of a cromlech can now be made out. -- Visited, 17th August, 1910.