The "Gewehr 91" was the name for the M88 carbine with a stacking rod added. Gew 91 were issued to foot artillery and other non-mounted special units. Your example was made by the royal Prussian rifle factory Erfurt. "n.m." = neues material shows it was made with the improved, reinforced barrel. It was first issued to 15th (Prussian)Reserve Artillery regiment on foot, 4th battery, rifle number 15. After WW1 started it was withdrawn from service and converted to use the then-standard S-type ammo, "S" stamp on receiver ring. The bolt body was replaced with one of another carbine, correctly renumbered . The magazine was fitted with the clip-ejecting cover at the same time. Then it was reissued to the 1st Reserve Artillery Regiment, 8th battery, rifle number 107. Here is a photo of a peacetime soldier with a Gew91: