Well done for tracking down those RFDCC references! They mentioned that the locations had been passed to "The Registry" - presumably the Cambrian Cave Registry?
http://welshjournals.llgc.org.uk/browse/viewobject/llgc-id:1171940/article/000042358 The two lead workings near Chepstow that Tucker mentioned are:
Lead workings at Piercefield near Chepstow (unlocated by Tucker; placed by him in 1km square ST 52/95) - see pp.38-39, 39-40, 50. Tucker referenced W.Rees "Industry before the industrial revolution", vol.2, pp. 448 and 483 (error for 484), both of which in turn reference "Llwyd, 1697". Tucker also referenced the booklet I.Waters "Piercefield on the banks of the River Wye", Chepstow, 1975, p.26 (a booklet I have not seen), which mentioned workings in 1784. Llwyd, 1697 is usually given as Edward Lhuyd. Rees's bibliography on p.674 shows that he consulted the published transcribed version of Lhuyd's "parochial queries": R. H. Morris (ed.), Parochialia being a summary of answers to "Parochial queries in order to a geographical dictionary, etc., of Wales", "Archaeologia Cambrensis supplements", April 1909, 1910, July 1911. The "parochial queries" were printed sets of questions that Lhuyd sent to parishes across Wales in 1696 to gather information on numerous subjects (including mines and minerals in which he a special interest) for his proposed publications on the natural history of Wales, for which he also toured Wales in 1697. He died in 1709 before publishing more than the first volume (on Celtic languages) of his ambitious proposed multi-volume work on Wales. The manuscript answers to Lhuyd's " parochial queries" are in the National Library of Wales; the 1909-1911 published transcript is the version that most researchers use. Rees merely states (p.448) that "Llwyd in 1697 speaks of the mining of lead ... at Piercefield near Chepstow", and (p.484) "he speaks ... of the mining of lead at Piercefield near Chepstow". Arch. Camb. Supplements published transcripts of original documents; I cannot find the relevant vols online.
Lead workings at Galley Hill near Chepstow (ST 530/936) in 1789 - see p.48. Tucker referenced this to “Gwent County Record Office E. & E. 0736”. Gwent Archives (as it now is named) does not appear to have an online catalogue.