My dad has three Mauser 98s. Two are Yugo 44 refurbs. All are in excellent condition from his fathers old extensive gun collection. The one I am looking at is a true German with mostly matching mostly matching numbers and the off ones match each other. It is a 1936 with a manufacturing code of S/42(I looked it up earlier and found the factory), has lots of eagle marks on many of the plates, but has two markings I am confused about.
The first is "Kal 7.62 M/M"

The second is one of the serial numbers on the upper is "FLY-the number-"

I assume(not fired) the 7.62 part means its chambered in 7.62, although the tag on the rifle when it was on sale at grandad's shop says 30-06.

Were any originals chambered in 7.62? What kind of 7.62 would it fire? x54, x51, etc.?