Yes, your Mauser has been sporterized. After hitting on your Avatar and seeing the other pics of the rifle it is clear that it has been reworked. If you know FOR SURE that it is still in 8mm Mauser, it should be fine to shoot the way it is. But there have been many, many Mausers rechambered to take other cartridges and some folks just didn't mark them as so. You would be very smart to have it checked by a gunsmith to be sure if you have the slightest idea that it may not be the 8x57 cartridge. As Nothernug stated, it has happened a lot over the years to sporterize the K98k Mausers. The value of it being an original rifle is gone but, the value of it coming from your Grandfather is there. Have it checked and enjoy!