Tungsten carbide slip gauge blocks. These are used as a standard form of length measurement. They are precision ground (lapped) to a specific thickness and then stacked to create the required length. The instructions inside the lid of the box refer to 'wringing', the process of placing two or more gauge blocks together so that the ultra-flat surfaces adhere to each other. A series of such wrung blocks will support their own weight. Photographed at the National Physical Laboratory, Teddington, UK.