By Harpur & Tunstall Ltd. 1940 dated. 'Capt. Field's Improved' boxwood parallel rule with brass fittings. Approximately 45.5cm in length x 7.5cm wide when closed. Clear Air Ministry and crown markings. A bit soiled with light scratches and a few 'bumps' around the edges but in good used condition overall. As used by the Navigator for route plotting in bomber aircraft. Stock code M22351.