That is neat! I picked one up, as well as an "Aim Corrector."
Swedish AK4 Aim Correctors - Like the one pictured, all are in nice looking used condition. Sweden adopted the German G3 rifle design and produce it as the AK4. The aim corrector mounts to the receiver behind the rear sight and has an angled smoked colored glass lenses that allows the instructor to observe the cadets sight picture during dry fire exercises and allows the cadet to see thru and align the sights on target.