The B2-Pro/L Screw-Knob Clamp from Really Right Stuff is compatible with most Arca-type camera plates, camera bars, and multi-purpose rails. It features a knurled knob for better grip, so you can apply more clamping pressure onto a plate. Unscrewing the know allows the jaws of the clamp to open wide for plates to be top-loaded quickly and easily.
At the top of the clamp it features three counter-sunk sockets. The center socket is non-threaded, and it accepts metric 6 (M6) and 1/4"-20 threaded screws to mount to a ball head stem. The same center socket is threaded 3/8"-16 on the bottom to allow it to screw onto a 3/8"-16 male ball head stem. Two additional 1/4"-20 threaded holes are spaced 45mm apart from the center socket.
The CNC machined 6061-T6 aluminum clamp has an anodized type II black finish. Index marks are laser-engraved on both sides of the clamp for repeatable positioning.