Toronto. When Leitz came out in 1954 with the wildly popular M3 camera, they took into account the money Leica fans had tied up in auxiliary lenses and accessories. While the M series camera bodies were thicker than the older screw-munt bodies, the actual film to lens mount distance was 1mm shorter and the diameter of the body opening for the lens slightly larger.
This meant a thin tube could be mounted by bayonet to the M3 and threaded on the inside to take a screw-mount lens! The tubes came in various styles and were cammed to automatically set the bight line viewfinder frame. Wrong adaptor? No problem! Just use the tiny lever on the camera front to change the bright line to match the lens. Even today (2018) an adaptor will allow older Leica lenses and accessories to fit a modern day camera with a Leica screw-mount style lens mount.
The various original 1mm lens adaptors are ridiculously expensive today!