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Amgueddfa Cymru – Museum Wales
National Coal Board. Certificate of Authority as a Competent or Authorised Person. Issued to David John Williams of Blaenserchan Colliery authorised to "carryout the duties of electrican in charge of the mine in the absence of the unit electrical engineer and assistant electrical engineer". Dated 1 July 1960. Photocopy of black print on white paper with details handwritten.
