The stamps on the right receiver were acceptance marks (the imperial counterpart to the WaA acceptance on k98k) The three pictured indicated receiver hardness, assembly and testing respectively.
The "B" stamp indicates Beech wood. As walnut supplies dwindled, other alternatives were employed as a test measure-- the known ones were "B" for Büche (or Beech), "R" for Rüster (or Elm), "A" for Ahorn (or Maple) Beech is the only one that is commonplace by wars end. Maple is quite rare and Elm was only widely seen on the 1918 Tankgewehr, though an example of an elm Gew98s exists. Other innovations, such as a two-piece stock were also used.
One small correction- it is a Kar98, or Kar98a -- the Kar98az is an anachronistic collector term that was only ever used to describe a specific trials model (the "a" and "z" were from the german terms for bayonet lug and stacking hook). Most have gravitated to the Kar98a nomenclature today. It was only ever the Kar98 in WW1 and the 98a term was devised in the interwar period to distinguish it from the Kar98b.