I was looking for a non-destructive solution to have the 35 mm framelines brought up while using my summicron-c 40 mm lens (which brings up 50 mm framlines by default). Jamming a piece of cardboard behind the frame preview lever was just too ugly a solution. Therefore, I designed this little thing to keep the lens preview lever in place. It seems to stay in place reliably. If it would ever pop out of place I may tweak the design a little bit more