Nice photo! You can see the leather panel on the leg of his trouser. Here is another one. Commercial photo titled "Vorposten vor Bayonville" (France) "Setting up outside of Bayonville". Bayonville is southwest of Metz in Lorraine, France. Photo was canceled to the Festungs - Eisenbahn - Bau Ko. 12, Metz, 8 Juli 1915. Card was produced by a firm in Metz.
Both mounted soldiers, wearing Kugalhelms, and both men are armed with Gew 91's.They must be Feldartillerie men. We can see Swedish cuffs on their feldrocks. There were two regiments stationed in Metz, 1. Lothringisches Feld-Artillerie-Regt. Nr.33, 2. Lothringisches Feld-Artillerie-Regt. Nr.34. (XVI Armee Korps).