ZWO C-Mount Adapter/Extension from C-Mount male to T-2 female thread
C-Mount Adapter / Extension from C-Mount to T-2 (M42 CS Adapter)
On the top of the image you can see the female C-Mount inner thread. At the bottom there is a male T-thread visible.
One use of this adapter is to connect photographic dSLR cameras to a microscope with male C-mount. You would also need a brand specific T-ring to connect your camera.
Another use is that this adapter will also convert a Baader C-Mount Camera Lens adapter into a T-threaded adapter.