ON M3170 is Oestereichische Norm „Laufstähle für Jagdfeuerwaffen“ = Austrian industry standard # M3170, barrel steels for hunting firearms, quality specifications. St(ahl = steel) Lf (Lauf = barrel) #3 (Whatever #3 means?). you may find the Austrian 1977 proof rules here:
http://www.ris.bka.gv.at/Dokumente/BgblPdf/1977_448_0/1977_448_0.pdf
To find out what StLf3 really is, you will have to buy the industry standard here
http://www.beuth.de/de/norm/oenorm-m-3170/7086288
or here
http://www.bdb.at/Service/NormenDetail?id=3619
As I am no metallurgist, I don't care about the exact specs. So I shy away from the cost.