The "Korsu" is a special version of the Suomi made for use in the bunkers of the Mannerheim Line. When construction on the Line really kicked into high gear in the summer of 1939, is was discovered that the vision slits in the bunkers were too small to fit the muzzle of a standard m/31 Suomi. In addition, the sights of the m/31 were uselessly located above the barrel. As a solution, the Korsu model was made. The frame was redesigned slightly into a stock-less pistol grip, and a new barrel shroud was made with a tapered muzzle that would wedge nicely into the standard construction vision ports. These new barrel shrouds also had simple fixed iron sights on the left side, making them usable when firing through ports.

The first batch of 500 Korsu model guns were made in the fall or 1939, in a serial range between approximately 5900 and 6400. A second batch of 500 more were made in January 1941, with serial numbers scattered between 13000 and 17000. They remained in Finnish inventory until the 1970s, and are very scarce today.