Not much to report I'm afraid. I took the lane north through Rhiwderin off the A468. There is no public right of way to the fort and visibility was difficult due to the very high hedgerows. I parked near the water treatment works and all I could see was a typical tree covered hilltop.
I've added two diagrams from 'Archaeologia Cambrensis' of a purported cupmarked stone in the area. http://www.archive.org/stream/archaeologiacam21assogoog#page/n277/mode/1up
Mr Thomas doesn't really give away the location, he mentions 'an old enclosure' but I don't know if it could be this fort. But it's not on Coflein as far as I can see. Dunno what people think or if they know more. Or maybe they are genuine and hiding under some turf somewhere.
I enclose a sketch of what seems to be a cup-marked stone which I observed yesterday near Rhiwderin, Monmouth. Unless there be some operation which simulates such markings with which I am unacquainted, I take the specimen to add an instance of these mysterious prehistoric remains to the very short list given for Wales by Mr. Romilly Allen, and to be the first reported for South Wales.
The stone displaying the cup-markings is a mass of millstone grit, earth-fast, the slanting surface appearing above the turf being about a yard wide, and 4 feet long. Upon the upper half of the surface is a group of twelve cups from 1 1/2 to 2 ins. in diameter, and about 1 in. deep. On first noticing the cups they were taken for holes out of which quartz pebbles, abundant in the local millstone grit, had been weathered, but examination of the block showed that no pebbles of large size exist, or had existed in it, and the conclusion was arrived at that the cups are artificial. On turning back some of the turf covering the base of the slope of the stone, no other cups were discovered.
The stone lies within an old enclosure, as shown by wild apple-trees and an abundance of daffodils, and still more clearly by ruins, which seem those of a cottage or small farm near by. This contiguity to a habitation which does not seem to have been abandoned more than a century, made me suspect some medieval or more recent origin for the markings. I cannot, however, account for them otherwise than by supposing them to be cup-markings in the technical archaeological sense.