ZWO M48 Male to M48 Female Adapter and Extension - 16.5mm Length
ZWO M48 to M48 Adapter with M48 Male Thread and M48 Female Thread (2" filter thread) - 16.5mm Effective Length
There are lots of practical applications for this adapter, but the main reason ZWO came out with this product is to create the correct (frequently used) 55mm back focus when used with the M54 to M48 adapter/extender (model: Z467-M54-M48-21) on their full frame cameras (i.e. ASI094MC-PRO and ASI128MC-PRO).
So you have a 17.5mm focal flange distance of the full frame ZWO camera, then you have a 21mm long M54 to M48 adapter and then you may either use a ZWO OAG that is 16.5mm long, thus makes up 55mm total back focus, or if you don't use a ZWO OAG, then you will need this adpater to create the correct distance, assuming that for your flattener or reducer you need to achieve 55mm back focus.
-- M48 male thread (2" filter thread)
-- M48 female thread (2" filter thread)
-- mechanical length: 16.5mm