This 7.4" MPR-CL II Rail with Integral Clamp from Really Right Stuff has an Arca-type double dovetail design that can accept accessories on either the top or bottom of the rail. The rail also sports a screw-knob style clamp on one end.
The MPR-CL II rail can be mounted on a ball head, used as a macro slider or focuser, or can have an orbital-style flash bracket attached to it. There is a laser-engraved scale with 1mm increments on both sides of the rail, for precise position repeatability. Additionally, a built-in bull's-eye bubble level is located towards the front of rail to keep the camera level.