I own two Otto Koch chinstraps; one with brass fittings, one steel. I have reservations about these and have never seen one that wasn't stamped 1917. Furthermore the "& Co" seems a bit too American for my liking. "Cie" is the abbreviation I would expect.
The leather is always extremely thick and the adjustment buckles massively flat and square. They are completely unlike any chinstrap I have encountered original to a Pickelhaube or Stahlhelm.