In his book The Spirit Paths of Wales (ISBN: 1-85284-289-X) Laurence Main states that, “two cairns stood here until 1910, when Council workmen destroyed the larger one and carted away its stones for road metal”. He also describes a Bronze Age axe-hammer found at the site, made from “spotted blue dolerite from Preseli”.
As ever, Main doesn’t include any of his sources, but I’ve read about the Council worker story somewhere else. Strangely, I discerned two cairns on my visit, so I’m not sure whether the smaller of the two was the ‘destroyed’ cairn of whether there were originally three cairns (probably the former).