By A.G. Thornton Ltd. 1940 dated. 'Capt. Field's Improved' boxwood parallel rule with brass fittings. Approximately 45cm in length x 7.2cm wide when closed. Clear Air Ministry and crown markings. The rule has minor edge bumps and doesn't lie completely flat but is in excellent overall condition. As used by the Navigator for route plotting in bomber aircraft. Stock code M26586.