This Garmin mount arm is specifically designed with the Garmin Varia in mind. It uses a backrest to prevent the typical rotation caused by the torque from the offset battery. A tough rubber ring is used to secure the Varia to the mount arm and prevent twist out.
The mount arm is designed specifically for Varia models RTL510, RTL515, and RVR315. It will also work with the original rectangular Varia, but the backrest ring will not be used.