Mick,

That there is a relative movement between the slit and the film, may the film move and the whole camera rotate, or the camera and film stand still and the lens swings, is part of the game for panoramic cameras beyond 110.

But as the slit has passed the film in a swing-lens camera, the lens either has to make a further 220 turn or swing back to be ready for the next exposure. During this movement the slit has to be covered. This is similar to cocking a common focal plane shutter where the slit between the curtains has to be closed or covered.

An alternative way would be to stop a swing lens after exposure just off the film, thus blocking the slit. Then advance the film, and expose by means of swinging back to the initial position. Again stopping just off the film.