The Angle Cosine indicator is a simple instrument used for shooting in mountainous terrain, up/downhill to obtain the corrected gravity distance to the target.
The "Mil Spec" variant is quiet, guaranteed water tight and made from aircraft grade aluminium which is anodised in flat black. The unit is completely mechanical so no problems with low battery or electronics failing. Being manufactured to US Military directive.
The cosine numbers are laser engraved at 5-degree increments.