Wonderful Double gun. It is likely one of the german / Bavarian cartrdges. The cast with ferosage is a good idea to start. If you have this, Rocky Mountain cartridge can make brass. A bit pricey at $64.00 per 20, but it has put my 9.5X56R in my combo back to shooting. My combo looks similar to your double rifle. These can be great guns. The actions (similar to jones underlever) are strong and stay tight with normal loads. Let us know more. I will post some of my photos and the building of loads.
You should get a copy of the book European Sporting Cartridges once you do the cast. You can ID your cartridge from the measurements. Some of these cases can be made from .444 marlin. Some from .43 Mauser, and others are actually derived from the .450 BP Express of the UK.
Good shooting once you get it going.