I think it is just a design preference by the individual countries that had ordered from Mauser. The straight bolt was vogue around the turn of the century (1900) and I believe the turned-down bolt came into use with the shorter barrel (23.5" vs 29") and the designation of carbine or Karibiner for cavalry use. I believe some countries, Spain, Argentina, Chile, Mexico, ordered turned-down bolts for cavalry use as they maintained mounted cavalry either into or just before the outbreak of WWII. HTH.