This is a military protractor calibrated in Mils – differing from the one in your school maths set which is in Degrees. On this scale, a circle is divided into 6,400 mils.
Using the military protractor, a bearing can be plotted from one point to another in order to determine the bearing. This tool has a useful thread attached to the cross hairs of the protractor, to assist in the determining of that bearing.
Romer scales are included for maps at:
The slight blue hue of the protractor in the photograph is caused by a protective film over the protractor – the actual tool is made from clear perspex.