These are split frames that replace the original magnesium frame of the Hirobo Lepton EX. The rear allows for the motor to be removed with the frame so making it easy to adjust gear mesh and tighten the motor screws.
These must be printed in powder nylon (MJF suggested) to provide the required strength and durability. FDM printing is NOT recommended for these parts.
To use the M2.6 screws tapping is strongly recommended and not just driving the screws in. If screws for plastic are available, 2.2 size can be used which provide better hold. In this case of course no thread is required in the frame.