Sitomètre Modèle (Mle) 1911 

No:  10258  Armée Belge 

This device is an artillery level. It is also called sitogoniomètre because it combines a sitometer and a compass. This device is used to aim pieces of artillery (setting of the elevation angle) at a target. It was invented in 1910 by Gilbert GARNIER, a French officer (1874-1964) while working at the weapons plant Ateliers de Puteaux (APX).

( source Compassmuseum.com)