During World War II, the Swiss military experimented with two models of K31 carbine with integral optics (the K31/42 and K31/43). These were found to be not sufficient for military service, and after more experimentation and development, the ZfK-55 rifle was adopted in 1955. What we are looking at today are a pair of transitional guns from the developmental period between the two. These two rifles came out of the SIG museum, and show a number of features with both the K31/43 and the ZfK-55. For instance, they use a prismatic scope like the K31/43, with the same type of range adjustment. However, the front end of the scope is fixed in place, where on the 43 model it could fold down for protected storage. Additionally, the scopes on these rifles are detachable, like the ZfK-55. Unfortunately, I have no information as to the exact dates of these two transitional models, nor details on their trial and evaluation. K31/42 & K31/43: https://www.youtube.com/watch?v=i7Gtn-OD6jg ZfK-55: https://www.youtube.com/watch?v=wAshu34OvC0