Schneider Kreuznach Radionar 80mm f2.9 TO M42 male adapter. RARE.  Use this to adapt your Radionar to M42.  I've adapted several to Sony E using this setup.  I recommend some red threadlock once you get infinity focus distance correctly set.  Attach this to your M42 camera or adapter on the camera side and Radionar on the other.  Also fits Baldax 80mm f2.9 lenses.  Don't ask me what size this is or the thread pitch.  I've long forgotten, but yes, it was extremely difficult to find.

Use this to create a removeable, non-permanent but completely solid mount for your Radionar 80mm f2.9 lens on a modern camera.

Example images I've made using this setup can be seen here:  https://youtu.be/5Djuk0CAMtI