Trans "Red Rock", its Ceredigion just down the road from Frongoch. 3907 is Pont Ceunant generating station (SN706742) and actually part of Frongoch, but may require an entry of its own. Graigoch is at SN704740
The site is much altered, years ago you could still find the timber dolly stands for the run of flat rods to the engine shaft, this now obliterated by track widening.