From Manufacturer
Acratech's Arca-Type Locking Lever Clamp for GV2 or Ultimate Ballheads will work with any quick-release plates that are Arca-Swiss compatible. The clamp features an adjustment knob to account for any variance between different plates. It has a precision laser-etched scale for accurate camera positioning. The clamp is also outfitted with a highly visible 15mm bullseye level to quickly balance the head. The locking lever and included detent pin help prevent accidental plate release. Weighing less than 4 oz, stainless steel construction and black anodized finish nevertheless give this clamp a solid feel. With 3/8"-16 and 1/4"-20 mounting thread adapters, it will fit virtually any tripod head.
This clamp cannot be installed by anyone other than Acratech. Your GV2, V2 or Ultimate Ballhead must be sent to Acratech for this installation. This service is free, but you will have to pay shipping costs.