Coming late to this thread but I hope this note may be of use...
The two mines featured at the start of the first post on this thread have short entries in J.R.Foster-Smith, "The mines of Merioneth" (British Mining no.6), NMRS, Sheffield, 1977:
Ty'n-y-ddol is mine no.10 on page 7, NGR SH 637/426 "A trial level was driven here in search of lead and copper ores in Ordovician slate. No reference to this working has been seen."
Bryntirion is mine no.11 on page 7, NGR SH 636 / 422 "A small mine or trial in Ordovician slate, probably worked for copper and lead ores, but no details of the workings have been seen."
Foster-Smith does not reference any source (including Bob Owen Croesor's book which is absent from his bibliogaphy), so he likely picked these workings up through his scan of 1:25,000 and 1:10,560 OS maps. "Old Levels" are marked on the 1:10,560 maps from the 1887 survey onwards.
The two mines featured at the start of the first post on this thread have short entries in J.R.Foster-Smith, "The mines of Merioneth" (British Mining no.6), NMRS, Sheffield, 1977:
Ty'n-y-ddol is mine no.10 on page 7, NGR SH 637/426 "A trial level was driven here in search of lead and copper ores in Ordovician slate. No reference to this working has been seen."
Bryntirion is mine no.11 on page 7, NGR SH 636 / 422 "A small mine or trial in Ordovician slate, probably worked for copper and lead ores, but no details of the workings have been seen."
Foster-Smith does not reference any source (including Bob Owen Croesor's book which is absent from his bibliogaphy), so he likely picked these workings up through his scan of 1:25,000 and 1:10,560 OS maps. "Old Levels" are marked on the 1:10,560 maps from the 1887 survey onwards.