This is a double-threaded adapter to screw a camera module (C-mount 1"-32 UN 2A) on top of the trinocular Amscope extension tube (M26/0.75) you can find on models such as the SM-3TZ-54S.

The OpenSCAD file is available and all dimensions are configurable, in case you need to adjust something. It's based on polyScrewThread_r1 library (http://www.thingiverse.com/thing:8796).
There is a "correction" factor of 1.005 in the file because the printed thread was a bit smaller than specified. If you've problems with the print, check the outer diameter of the M26 thread, it should be about 25.85 (comprised between 25.73 and 25.97).

To install it, unscrew the silver tube from the extension tube and screw the adapter instead.
Screw it gently to its maximum.
Beware both threads are very similar so take care to put it in the right direction.
The large thread is the one that goes into the extension tube.
The small thread is the C-mount.
Once settled, adjust focus for your eyes, then switch to the camera and adjust camera focus with the extension tube, so now you can get both eyes and camera focused simultaneously.
To adjust the image orientation, free the base of the extension tube (there is a small screw on the outskirt) and rotate it.