This a mid-15th century wrought iron breech for a breech loading cannon retaining both its original powder load and wooden wadding plug. In the case of breech loaders, the ball was shoved in the barrel from behind manually before the breech was put in and plugged by a wooden wedge at the rear.
Of course, this 15th century rapid firing method only made sense with a number of loaded breeches right at hand.
Actually I posted these before in my thread "Breech Loading 1450-1550" but given the fact that this rarest stuff is hard to grasp, and even harder to remember after long time passing, I hope not to be blamed too severely for a little repetition now and then.
Best,
Michael