Unlike the larger cairn occupying the summit of this rocky ridge (SN879867) CPAT [PRN4960] reckons this cairn probably represents additional field clearance upon an original monument:
"Poss cairn some 7m dia built around natural outcrop may be natural feature (CPAT site visit, 1978). Area now afforested site unlocated - prob destroyed (OS, 1981)....Site is now occupied by a modern clearance cairn. Probably an ancient site reused (Gibson, A M , 1998 , Prehistoric Funerary & Ritual Sites: Upper Severn Valley)."
I was actually looking for the nearby Pen-y-cerrig cairn shown on the map a little to approx north-east. However I failed to positively identify that one - make of that what you will - but stumbled upon this instead as compensation. That'll do. Some great views, too, particularly toward Pumlumon.