I assume that Gew. 88 is on the left side of the receiver, and V C Schilling, Suhl, is on the top of the receiver ring.
If that is correct, it is a Model 1888 German rifle, usually called the "Commission Rifle" because it was designed by an army commission using what they considered the best features of several designs being considered. It is sometimes called a Mauser, but that is not correct and it has little in common with any Mauser design.
The original caliber is what we call 8mm Mauser, BUT it was the old cartridge with the small (.318" diameter) bullet. Unless the top of the receiver ring has an "S", the correct ammunition is what is called the 8x57j, a cartridge that is not readily available any more. It should not be fired with modern (.323" bullet, 8x57js) ammunition or any 8mm surplus ammo. If the "S" is present, modern or surplus ammunition may be used.
If the rifle is unconverted, it must be loaded with a 5-round "en bloc" clip, which falls out the bottom of the action when the last round is fed into the chamber. The clip is easily recognized by the large round hole on each side. The conversion included not only the "S" mark, but the installation of a stripper clip guide and a device to allow the gun to be loaded from standard Mauser 98 clips.
Jim