Thanks Jim, the article clearly outlines how lethal the shotgun was with a 9 foot width and a 3 foot vertical spread was enough to take out multiple personnel with penetration of a 2" plank at 150 yards from the muzzle with plenty of energy remaining to do damage.
From my foggy head in an article long ago it stated with a good few of these in a trench having 5 cartridges in each shotgun loaded with 9 pellets in each round and plenty of cartridges at hand they inflicted grievous losses on the waves of advancing Germanicon soldiers.

So here are the pics of the cartridge holder I have as I am not sure if it is WWI war related or an early sporting rig the only markings on the leather is on the waist belt tail and just says 50 !
On the brass shell holders of which it has 20 are the following;

PAT APR 16 - 78

P & J SERENA ILL (Which I surmise stands for Illinois)

Note ~ the shell holders also have a bottom plate so the shells cannot slip and fall out the bottom of say from a loose holder.