This ZWO adapter has an optical thickness of 21mm. On the telescope and camera side it has an M42 thread also known as a T2 thread.
It has an external diameter of 2" so it can be used as a 2" to M42 (t2) adapter to connect DSLR rings to 2" draw tubes or focusers.
|Telescope connection||M42 x 0.75 female thread (T2)
M42 x 0.75 male thread (t2)