They are called Kaiserhaken (Kaiser Hooks) allegedly developed for the Kaiser as he was lame in his left arm after a disaster at birth. These hooks were designed to allow him to use a tree as rest to compensate for not having a left arm to support the forend. The hook pivots out of the forend and is gently pushed into the tree on the rear side, the recoil will just pull the spike out of the tree. The steeply curved pistol grip called a Kaisergriff (Kaiser Grip) was also made for one handed shooting for the Kaiser to give more control for his right hand. He also had shotguns which had button triggers on on top of the grip where the safety normally is so the gun could more easily be fired with one hand.

Matt.