This is just a fixture for testing motors and shafts that will be used on the xb-35. Basically just a box that will mount on the thrust stand shown by the link below. It also details one use of the magnetic building board.
I just use small c-clamps to hold the motor, shaft, and bearings so they can be vibration tested.
The building board is shown in detail in the following thread:
Most of the motor assembly can be seen in the pictures attached however I did not take any photos while the motors and shafts were in the test box.