The sliding clip that fits over the rear flange on the AHRS and locks it down tight. The 2 nut plates have hexhead screws up from the bottom that lock it down against the angle. The angle is slotted to allow the clip to slide away and give enough clearence to lift out the unit. The black area at the top of the picture is a velcro pad to mount the Battery for the Garmin 396 Claude Stokes 5/26/2007