Coflein has parts of Bwlch y Plwm as being possibly Roman, most likely top of the hill, and indeed there is a Roman Level. And Bick states that an old smelting hearth and large lump of lead was discovered on the hill in 1850.
Speaking of which, it's a bit odd that 'Old Copper Mines of NW' has such a fleeting mention of BYP due to it being lead and not copper. Surely it has a greater affinity to the Snowdonia copper mines than to the mid and north Wales lead mining areas.