Original files and updated info: https://www.thingiverse.com/thing:4664097

The demo video shows one side of an Anet A8 printer, of course the indicator ring is mounted on both sides : https://youtu.be/3GsEOtJSWuk
https://youtu.be/e03xUaLFAO8

This is an indicator tool for checking the synchronization (aka both sides the heights are even) of the two Z stepper motors at any time, even during printing. Thus, it can also be detected if one of the stepper motors has missed any steps or the steps sizes (layer height) are uneven. After homing, it shows the accuracy of alignment and if necessary the correct position of both motors easily adjustable again.

For printers with single motor Z system, this indicator also can be useful because it indicates the real level on the Z axis compared to the shown value on the printer display.

If the scales and the rings are correctly mounted, the resolution of height indicator is 0.02 mm (or with good eyes and estimating about 0.01 mm). With this resolution, one can verify whether the printer is able or not the fine height positioning evenly on both sides over the full range of Z.

This indicator is designed for printers where one turn of Z stepper motor (lead screw) lifts the head by 8 mm. This is a common 3D printer hardware setup many printers are built this way. The ring inner diameter is 19 mm, fits the most popular aluminum couplers. However, this size of the couplers is not critical, so they are manufactured with a fairly large tolerance. Use the scaler tool in your slicer accordingly to get the proper sized indicator ring.

Why do we need it?
The dual-motor Z system easily loses its alignment to each other when the printer is off or the electronic disables the stepper motors (goes to idle). Any touching of the printer for example replacing filament, cleaning, etc may lead to skewing of the X-gantry in the XZ plane.
If this misalignment is not noticed, the extruder may rise at an angle stressing the printer mechanism or causes uneven layer heights, weak layer adhesion and spotty surfaced print (nozzle is not perpendicular to the bed). The skewed / tilted X-gantry is more sensitive to bent Z lead screws, causing periodic jamming during lifting the printer head what leads to inaccuracy of the fine positioning. It may even be stuck somewhere at any height. The skewed system also leads to faulty bed leveling.

Some videos for the better understanding:
https://youtu.be/8k2pWQ4To8s
https://youtu.be/dBP7pHOG7X0

The ball bearing trick (red ball) used in the lead screw coupler is not necessary for implementing the indicator ring. However, I find it useful to eliminate the backlash and unwanted flexibility in direction Z of the coupler, but the coupler's ability to compensate any misalignment in the XY plane (deflection, not lined up, etc) remains.