http://www.rockislandauction.com/viewitem/aid/66/lid/1536

The Swiss military began experimenting with scoped sniper rifle during WWII, with the K31/42 and K31/43. These use periscopic optics permanently mounted to the side of the receiver, and were both found less than ideal. Experiments continued after WWII, and the periscopes were replaced with tradition style scopes on quick-release mountings.

Eventually the idea of making the sniper rifles mostly parts-interchangeable with the standard K31 carbines was also discarded, and the ZfK-55 was adopted. It uses the same basic action as the K31, but only a few small parts can be interchanged between the two types of rifle. The ZfK-55 has a longer and heavier barrel, heavy stock, integral bipod, muzzle brake, and most unusual of all, the action is canted slightly clockwise. This was done to allow the optic to be centered over the barrel but still allow the use of standard 6-round charger clips to reload the rifle.

For my video on this rifle's predecessors (the K31/42 and K31/43) go here: https://www.full30.com/video/79deed9c401781e6ba9dd8694570a00a