I believe this is the YELANGU D100 Shoulder Mount Rig with Matte Box
The top handle rod is missing (although the handle is there).
"Camera shoulder rig mounts provide stability, comfort, and balance for handheld shooting by distributing the camera's weight across your shoulder and body, allowing for smoother, longer takes with professional results, especially for documentary or run-and-gun styles, and offer modular ways to attach accessories like monitors, batteries, and follow focus systems. They create a more intimate, grounded feel, contrasting with gimbal shots, by using your whole body for stabilization."
This is sold as it. Everything tightens up and it was used only once. But as noted, the top handle rod is not included.