Deckungswinkelmesser 'JDEM Wedel H/6400'

A nice Deckungswinkelmesser tool, which was a vital device for calculating indirect fire by measuring differences in altitude over the terrain. One of these would have been stored within the relevant pouch on a mapcase. 
 
This particular example is an early example, given that it is made from brass, and is also fully functional. The maker's code 'JDEM' for J.D. Möller G.m.b.H., Wedel, as well as the serial number, is also inscribed.