This adapter allows using of easily available microscope objectives with RMS/DIN thread on camera with C-Mount (like some surveliance cameras, for example).
Difference from competitor adapters is second flange. With other adapters, you need to install an adapter onto an objective, and scres such assembly into a camera. When you trying to unscrew an objective, there is a chance that the adapter will remain in the camera mount, and if you'll try to screw other objective into the adapter remaining in the camera, there is a chance that it will fall inside the camera at all. With my adapters, you are relieved from these risks. It doesn't matter if you screw the adapter onto objective or into camera first. My adapters have raised part that alows screwing into the camera till hard stop, and not further.