This is the Stretch X arm for 5" Phoenix frame, with chamfered edges and cut along hte waeve for durability and stiffness.
In this revision, the screw slot has been replaced with a simple screw hole to protect against large torque loads acting around the outer arm screw from tearing the inner bolt slot away. While this means it now requires the removal of two screws to replace the arm, it improves durability in high-speed side-impact collisions with the ground or solid objects. The motor shaft hole is also smaller to strengthen the motor bolt pattern even further than in v1.1.