Originally Posted by Roger Hicks
Sliding copier. Put lens on bottom, camera on top. Slide one way to focus on screen (here on the right); slide the other way to substitute the camera for the screen. Google LEICA FULDY.
The various FULDYs were not specifically slide copiers. They were essentially a device that, when combined with an appropriate Leica body, functioned as a "sliding back" similar to those seen on large format cameras.
They were used to for various copy work, when combined with a copy stand, and to fit Leica bodies to microscopes and other optical instruments.
I can't see the OP's posted image, but the VFs often shown attached to a FULDY were not part of the basic FLUDY, but a separate accessory. There were various such finders and as I understand it these finders also fit the early Visoflex devices.