I'm tracking the British Army has 3 distinct families of artillery clinometers (Large, Inspectors, and Field) plus a mortar family too. Furthermore, the British Army has a mountain clinometer, Mark I B.L. 10 P.R. Clinometer. Large clinometers are used with coast, super heavy, and anti-aircraft artillery. Inspectors clinometers are used by inspecting officers for checking artillery systems and the inspectors clinometer are more accurate than a large or field clinometer. Field clinometers are used to lay and check elevation on field guns and howitzers. The British Army started World War I with Mark III Field Clinometer but continued to improve the field clinometer design during the war and produced 3 improved models: Mark IV Field Clinometer - 1915, Mark V Field Clinometer - 1916, and Mark VI Field Clinometer - 1918. The Mark VI Field Clinometer was used all the way through World War II. What book is your photo from and could you please post a photo of the cover? Thanks