This fun contraption is the U.S. M1903 Rifle with a fold-open stock and the Guiberson trench mirror sighting system.
Several countries during World War 1 tried out different varieties of special rifles and rifle mounts to shoot over the tops of trenches without exposing the soldier to return fire.
The Guiberson system was convenient because it was more portable than most and folded into a standard-looking 1903 stock configuration.
But while it was tested, it was never put into production.