By Harper & Tunstall Ltd. 1940 dated. 'Capt. Field's Improved' boxwood parallel rule with brass fittings. Approximately 45cm/18" in length x 7.5cm/3" wide when closed. Clear Air Ministry and crown markings. Some scratches and relatively minor 'bumps' around the edges, the largest photographed. Good + used condition overall. As used by the Navigator for route plotting in bomber aircraft. Unusual manufacturer. Stock code M21886.