yes, the Vorratsbeschuß proof mark thats guns get in 1893 when the german proof law came into action. to be clear that was for guns still in the racks of gun dealer to show they were there before the law and don't need the new real proof.

what you have is one of the rare double rifles for lancaster bullet cartridges. lancaster means here lancaster shotgun cartridge or center fire cartridge in contrast to the lefaucheux pinfire shotgun cartridge.
the bullet cartridge use a shortened shotgun shell with blackpowder load and a heavy bullet. such guns were outdated in 1893 but some still around.
you best way is magtech 24 ga shotgun brass, thats what this gun was made for. common case length was 40 mm but also some in 35, 33 or only 30 mm long. in no way a german gun for the .58 roberts.
the bullet was the so called cats head - Katzenkopfgeschoß.

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