I ran across a thread in the cartridge collector's forum that seems to point to the Hs. marking on the box being correct for Rheinische Metallwaarenfabrik in Sommerda.
https://forum.cartridgecollectors.or...-belgium/15266
The majority of the rounds in the box are headstamped RM and 18 so they are most likely correct! Yay!
-Jason