Paul Mauser found that the german steels of the time (early 1900s) were of an inferior consistancy. So he ordered his steel in Sheffield
There are several lists of steel types used by the Germans, Swiss and post war Mauser company, also an interesting discussion between the German army and Mauser about simplifying production and using lower quality steel during WW2. Basically, Mauser (August Weiss) told them to p*ss off
I've attached a document with an overview of Swiss, Wartime Mauser and postwar Mauser material types used in production.