This stereo viewer was made as a companion to the Verascope F40 camera (which was also marketed in the USA as the Busch Verascope F40). The viewer is very rare, as most F40 cameras were bought with the more common non-transposing viewer.
Because this viewer transposes the image pairs it is possible to view the stereo photos directly from an un-cut roll of film. In the accompanying photos it can be seen that a roll of film is intended to be placed in one end of the viewer, and threaded through a film gate (much like those found in a camera), and out the other end into a take-up roll of film. A knob at the back of the viewer advances the film - which engages with a sprocket in the middle of the film gate. With care it looks possible to avoid scratching the film.
The viewer uses natural light as a source for illuminating the slide, but a novel feature is that the light enters the viewer through a diffuser in the top of the body (rather than at the end as in other ambient light viewers). This is very convenient as most light sources are overhead, so it is easy to maximise the amount of light enteringthe viewer.
Once the light enters the viewer it passes through the film in the gate of the viewer, and then enters a penta-prism (one for each lens), which reflects the image from the film through a right angle, and flips the image so that it can be viewed in the same orientation as the original scene (remember that the camera records an image flipped upside down, and flipped left-to-right, making direct stereo viewing impossible). This novel design feature has the effect of transposing the original film images so the stereo photo can be viewed without cutting the film and mounting into slide mounts.
The viewer has focussing and lens-spacing adjustments. The focal length of the lenses appears to be about 50mm, with a diameter of 25mm, and the film gate is designed for viewing European format stereo images.