I've posted this over in the Imperial German section, but since it's a French gun, I thought I'd start an artillery thread here... I've more to add later.
This one is a breech view of a de Bange Canon de 155 L modèle 1877. Although a bit antiquated, this gun was in widespread use by the French at the beginning of the war and modified in 1916 for further service. This one was captured and in use with Bavarian Fuss-Artillerie, a common occurrence since the Germans respected the gun's range.