By A.G. Thornton 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. Light scratches and a few minor 'bumps' around the edges but in good '+' used condition overall. As used by the Navigator for route plotting in bomber aircraft. Stock code M22891.