the short green case? no real idea, dont make sense
short blank cartridges were made once with maybe 30-40mm long shells. a short "bullet cartridge" will not be right because this is clearly a birds gun and not some kind of big game killer.

if pinfire cartridges are dangerous because of the pin??? thats a good question but I never heard of any accident because a cartridge fell on a hard surface. they had a mistrust against this cartridges once because old catalogs before the great war told you pinfire cartridges were not ship by post but only on railway to your next railway station where you had to pick up the ammo. but loaded shotgun cartridges were not the norm anyway. this was the gun for man's growing up with muzzle loader so they bought primed shells and load the ammo at home.

btw, I have load and shot 16 ga pinfire for years and you will be certainly more careful while handling these.