I wish I had an explanation, but as I said, the DH is an enigma. Your photos illustrate four distinct styles of DH stamps, and the lazy S DH stamp on the GEW98 differs from the one on the DWM. My example is yet another variation.
Darned if I know who put them there or when. Except for yours, all the DH Lugers I have seen have been dated from 1913 to 1918. If I were to venture a WAG, I might suppose that your GEW98 saw service with a Weimar era unit and was later employed by the SS. Quite a coincidence that your rifle would have seen sevice with two different DH units, but who knows?