Oops, sorry. It's a cartridge box (with sling)
Giberne is the French word. In Dutch it's 'Patroontas', though we use the French word just as much, which is why I didn't realize it's different in English.
Apart from a cartridge box, it's also a carbine sling. Not that I have a carbine, though. I do have to get the carbine ring, because the giberne is simply not complete without it.