The Landmark conversion refers to a modification of the Norwegian kammerlader rifles to allow them to fire rim fire ammunition, as opposed to being caplock weapons. It was invented by Jens Landmark.
The mechanism of the Landmark cartridge conversion is brilliantly simple. The chamber, which on a kammerlader is a separate piece tilted up and to the rear, is opened as before, but can be tilted further backwards by means of a hinge in the middle of the chamber. The 12.17 x 44 mm rim fire cartridge is placed backwards, facing the shooter, before everything is rolled back forward. The only part to be modified was the chamber and a curving firing pin was added where the nipple for the cap had been. Pictures showing the Landmark conversion can be found here [1]