K98k Calibre stamps indicating the land-to-land bore diameter in millimeters as 7.90, 7.9, 7.91, 7.92, and 7.93 are the most common, and a small number 7.94.
This was stamped (Calibre Designation Stamps) in front of the receiver from 1934 to 1942.
* See page 74, BBOTW.