The Zacuto Z-ZR3 3" Z-Rail is part of the QR Handle, EVF Top Mount and the Z-Rail Z-Lock. It is compatible with the RED Epic/Scarlet, Sony F3 and FS700, Panasonic AF100, Black Magic Camera and others. It attaches via (2) 1/4" 20 screws (included) that are spaced 3/8" screw center to center at their closest to 2" 5/32" at their furthest.
The 3" Z-Rail can be used on a second camera so you can quick release any of the Z-rail accessories mentioned above back and forth between cameras. The 3" Z-Rail has a 1/4 20" screw mount in the center to add a Noga arm or other accessories. You can add the Z-Rail Slide Mount for multiple accessory mount options.