WMS mentions two Nant Y Plwm mines. The one I was querying (and the one Morlock linked to) is closest to the village, close to the road bend and in the woods. Not much on that one.
When it was mentioned, I was told ' as a boy we played in the mine... it went in for half a mile 😉 O how things look bigger when your young!
Sinker... I've emailed you the N.letter as a pdf.