The British word for that would be a ‘coach key’ or ‘coachmans key’.
Quite common here, originally for flush fitting catches on horse drawn coaches, but later on you found them on railway carriages, motor coaches and some cars as well. Still find variations of them on the railways - you see the guard (conductor) on some trains carrying one in a leather sheath on their belts.
The keys are quite common, and someone probably still makes them.
The addition of a folding coach key to a ‘horsemans knife’ makes it a ‘coachmans knife’!