This part clips on to the back of a NEMA 17 stepper motor and provides an anchor for a zip tie to secure the motor cable, providing strain relief.
The “full” version fits the LDO stepper motors on the Prusa MK3, and also fits current Geeetech motors. I also have some very old Geeetech motors that have a different back plate which is the same depth all around, instead of having the thicker reinforcement at the corners - the “thin” version fits these old motors.
The author marked this model as their own original creation.