"Gewehr 91 is really nothing more than a Karabiner 88 fitted with a stacking hook underside of the fore-end just aft of the nose-cap. Issued to Artillery / Artillery support units, Train & Luftschiff troops. It was designed so artillerists could stack them keeping them out of the mud. The rifle was adopted for Prussia, Saxony and Wurttemberg on 25 March 1891. They were produced between 1891 - 1896 by private contractors, mainly C.G. Haenel or V.C. Schilling, and the Erfurt Arsenal. Those manufactored or rebarreled post 1896 were Z modified (deepened rifle grooves), and most were "S" modified between 1903 - 1905."

http://comissionrifle.blogspot.co.nz/2007/11/gewehr-91.html