The Gewehr 1891 was the last variation on the 1888 Commission rifle. It incorporated improvements for feeding from strippers IIRC. It is similar to the Argentine 1891 rifle. Your has been modified to be a single shot, from what you say, not uncommon before WW1 for rifles used for hunting.
What is it chambered for? That would be a wild guess. You will have to have the chamber cast and determine what the chambering is from Cartridges Of The World or a similar reference. There were a nmber of fairly popular (at that time) small-bore cartridges it could be chambered for.