According to Ludwig Olson..."Mauser Bolt Rifles"....I quote..."Restrictions imposed by the Versailles Treaty after WWI,prohibited the German manufacture of 8x57 arms and ammunition since this was the German military size. Consequently this caliber was discontinued in Oberndorf Mauser sporters but was resumed during the mid 1930's. Although an excellent caliber, the 8x57 is handicapped greatly by in that it exists in two different sizes: small-diameter bullet known as the Model 88, and a larger .323" size called the S type. If production of barrels for the .318" size had been dropped with the introduction of the S type much difficulty and confusion would have been avoided. However many German gunsmiths and hunters had the mistaken impression that the smaller Model 88 .318" diameter bullet (often called the "Normal" caliber) was more accurate, therefore the the Model 88 or "Normal" version was retained for sporting rifles long after the larger diameter S type had been standardized for military purposes." ....many 8x57 Sporters with the Model 88 rifling dimensions (.318"), especially those made by Mauser in Oberndorf, were marked "8x57 Normal".
So it would appear that you have an original Oberndorf Mauser sporter manufactured after 1930 made to shoot the small diameter .318" bullet.
The ammo is available but is a little more difficult to find.
Without seeing a picture, it appears that you have is a very high quality pre-WWII sporter that if in original unaltered condition is quite collectable.