This is the CURTAIN SHUTTER which is in the camera body just in front of the film plane.
A camera mechanism that allows light to expose film by moving curtains in front of the film plane.
a shutter that is positioned just in front of the film (Focal Plane). The exposure results from a slit traveling at a constant speed across the film - the actual shutter speed depending on the width of the slit. How the Shutter Works