Bronze Age Gold from Wales

The letter is typed in black ink on off-white paper. British Brown-Boveri headed paper. The graph is handwritten in pencil on grey-lined graph paper.
A three page letter from British Brown-Boveri Ltd to E.Ivor David regarding a paper on air compressors by a Mr Von Schulthess. Also includes a graph showing the comparative efficiency curves for turbo compressors with anti-pumping systems. There is a clip holding the four pages together in the top left hand corner. Loose within a beige/light brown file containing copies of the 'Metcalf papers' and documents relating to the 'Conversion of Turbo Compressor to Electrical Drive'.