Bought it from Knight with my first MK 85 back in the 80's. The stud on the barrel fits in the whole and holds the barrel in place for removing the breech plug. I haven't seen one advertised since I bought it. Thought it was kinda cool when I bought it. And it does work. Probably not necessary? Actually have the receipt that I paid $25 for it. Maybe it will end up being kindling in the fire place but thought maybe some of the Knight Muzzleloader enthusiasts on here may want it?