The K98k was the standard-issue German rifle of WWII. It was a direct descendant of the WWI Gewehr 98 Mauser rifle. Chambered for the 8mm Mauser round and fed by a 5 round internal box magazine, the K98k saw action on every front of the German war effort.
This particular example is a bolt mismatch gun. The gun was built by Steyr and matches itself while the bolt is from a different gun. The 660 receiver code is a predecessor to the later “bnz” code found on many Steyr guns. Rifles coded 660 are tough to find in nearly any condition.