It may be the same as one I called Cwm y Ffynnon in a short description in British Mining Memoir No 41 (1990) page 49. It is on the south slopes of the Tal-y-Mignedd valley south of Drws-y-Coed an on OS mapsheet Caernarvonshire XXLSE (1888). There was a mine truck in there in the late 1980s and a nice exposure of the vein at the end of the upper level (photo and plan in BM 41.