The Yamaha VCSB-L1 is a support bracket for vertical VXL series loudspeakers, allowing for downward angle adjustments.
From the Manufacturer:
The VCSB-L1B is a support bracket for vertical VXL series loudspeaker coupling with downward angle adjustment capability. This bracket is compatible with the VXL1-24 and 16 models of the VXL line array speakers.