picture indicates the anvil is part of the case. i don't see how any more gas would escape from this setup than would escape from the nipple of a frontloader. actually looks like a neat setup for backup under the circumstances.
we tend to think of the stuff paying sportsman and professionals use but back in the day there were a lot more locals shooting birds and buck for the pot than we might think. and they weren't using best grade english guns to get it done. back in '81 i spent a summer in zaire/congo with missionaries. one thing very easily found was black powder and muzzleloading stuff. brass cased rifle ammo not so much. shotgun shells could be had. i don't know the details and such, but i can only imagine a guy would benefit from any and every trick gizmo he could assemble in some of those areas where ammo is scarce. so having a box or so of those french shells would come in handy.