Royal Air Force 'AM' Marked Navigator's Parallel Rule

Large size by AG Thornton Ltd. & Co. 1940 dated. 'Capt. Field's Improved' boxwood parallel rule with brass fittings. Some tarnishing of metal parts, minor chips om the bottom left side. Good used condition overall. Used by the Navigator for route plotting in bomber aircraft. Measures when closed 18' x 3 1/8'