A clockwork mechanism consisting of identical overlapping metal blades
Most fixed lens rangefinder cameras use what is called a leaf shutter; a shutter consisting of very fine overlapping metal blades (leaves) which open and close in order to expose the film for a specified period of time to capture an image. The shutter is a clockwork mechanism and is powered by a strong spring tensioned by setting the shutter speed and charged by advancing the film before each exposure.
The manufacture of these shutters is highly specialised and camera manufacturers do not make their own. Shutters most commonly found in these rangefinders were made by Japanese companies Citizen, Copal and Seiko.