The Mauser Selbstlader M1915 was the result of many years of work by the Mauser brothers to develop a semiautomatic rifle suitable for military use. They tried many different types of operating systems, and this one is a particularly unusual recoil-operated mechanism.
Only about 600 of these rifle were made, with about 400 of them being shorter carbine variants and the remaining few hundred infantry-pattern long rifles like this one. They use 25-round detachable magazines (which look like MG13 magazines but are not interchangeable with them), and are chambered for standard 8mm Mauser ammunition.
The locking system of the M1915 uses a pair of large flaps very much like the Mauser 06/08 pistol, but not a recoiling barrel like the pistol. Instead, a camming plate floats on a bit of spring tension in the rear of the receiver. Upon firing, the plate tends to stay in place because of inertia while the rest of the rifle recoils backwards. This differential movement makes the camming plate go forward relative to the rest of the weapon, and in doing so it unlocks the two flaps. Very unusual.