ZWO M54 Male to M48 Female Adapter and Extension - 21mm Length
ZWO M54 to M48 Adapter with M54 Male Thread and M48 Female Thread (2" filter thread) - 21mm Effective Length
This adapter is compatible with the full frame ZWO cameras, but most likely there are some other practical applications can be found for this adapter as well...
The M54 threaded end is for the camera side and the M48 end is for the telescope side. Frequently that end would attach to a field flattener or reducer.
There is a reason for it being 21mm long. When screwed into a full frame ZWO camera (ASI094MC-PRO or ASI128MC-PRO), together with a 16.5mm long ZWO OAG, a frequently used 55mm back focus can be achieved. If an OAG is not being used, an optional 16.5mm M48 extension is also available (Model: Z464-M48-EXT16.5).
-- M54 male thread
-- M48 female thread (2" filter thread)
-- mechanical length: 21mm