I have something fairly similar, also a snap-lock, under lever, presently at a gunsmiths for a year, so it's a bit hard to be exact without it in front of me. It's a Roux breech too which does have that horizontal block referred to. It's of escentially the same caliber, 9,3x72R-Deutch which explains the 8,7mm stamp on your barrel. It's also a Drilling too hence the proof stamp G and crown, (G= Gezogen / rifled).

You may find another pair of initials inside the action. In my case it's RH which appears also on the barrel just behind the foreend hook. Perhaps Richard Hengelhaupt a barrel drawer at Zella-Mehlis. If your barrels were made between 1911 and 1939 in Zella-Mehlis I'd also perhaps expect to see a Z and an M emtertwined on them, as appears on my Drilling.