The workings were surveyed by myself and my father in the late's 1970's, except for the deep adit section which we briefly entered; soon after the entrance became completely buried. The fine portal of dressed stone included a date stone (1895 I think) but all this was removed about this time. There were quite a few artifacts remaining in the higher levels, all no doubt now stolen, & evidence of recent geological mapping (painted survey marks etc).
At the time the Ffestiniog Railway were driving their new tunnel to divert from the old one lost by the CEGB reservoir; the farmer seemed quite annoyed by the fact that Cornish miners (I believe under Bob Le Marchant) were doing the work when there were plenty of unemployed local slate miners available, who he thought would do a much better job.
Incidently the 1890's-1920's working was definitely for zinc.