The OS 1:25000 shows two cairns on Mynydd y Llyn in "antiquity" script, one to the west of Llyn Barfog (the NGR for this site) and one on a rocky eminence to the northeast of the lake.
Coflein and GAT only record the western cairn, but the GAT record states "Not visited but will be. This map square seems to have been missed off the database enquiry for the desk top study and this site only found during hand checking of all the map index sheets."
On visiting, the western cairn looks like a definite Bronze Age example, but the northeastern cairn appears to be a modern marker. It would be interesting to know the background to the OS choice of script for this one.