By A. G. Thornton Ltd. 1942 dated. 'Capt. Field's Improved' boxwood parallel rule with brass fittings. Approx. 45cms/18" in length x 7.5cms/3" wide when closed. Full Air Ministry and Crown markings. Some minor loss/chips at the edges. Good used condition overall. As used by the Navigator for route plotting in bomber aircraft. Stock code M17986.