Last night I pulled out my Borg 45ED II to see if I can make it even lighter and to give a bit more back focus. Currently, it is connected to a JMI NGF-S for fine focusing, and the connection is done by a Borg M57 to 2" visual back, a 2"->SCT adapter plus a SCT adapter on the NGF-S. It took around 1.5cm of back focus. To simplify this chain of adapters, we need an adapter like the following:-
The M57P0.75 male thread (4mm of thread) is to connect to the Mini-Borg tube directly, and the other end is to be inserted into the NGF-S directly.
An updated drawing:-
The blue part is the M57P0.75 thead which is 4mm long, the red part is the focuser body to show how it's fixed inside the focuser, there are 3 set screws to fix it on the groove of the adapter, and the focuser (red part) will serve as the stop to the M57P0.75 thread, so no need to make one on the adapter.
See below the original adapter has 56mm on the outside, and 50mm barrel (to the inside of the focuser) is not needed, for it does not help to fix the adapter inside the focuser at all.
Also, when the adapter sits inside the focuser, the M57 thread will have a natural "stop" so that no "stop" has to be made on the adapter.
For the PST etalon module adapter, I found that the PST visual back (eyepiece holder or blocking filter) can be screwed into the PST etalon adapter's 1.25" port if proper thread is machined there, and it won't affect it to accept a BF10 if needed.
after making the matching thread, they will be able to screw together:-