The Jobu Design QRR-138 is an Arca-Swiss compatible Quick Release Clamp suitable for replacing the old square, rectangular, trapezoidal or otherwise proprietary clamp on your ballhead. This Clamp is 50mm (2") wide and features one (1) standard countersunk hole that will work with either a 1/4"-20 Flathead bolt or a 6mm Flathead bolt. The QRR-138 features a 3/8 Tapped hole for installing into studded ballheads. The bottom of the clamp is slotted 0.260" wide by 0.790" long ( 6.5 x 20mm)