The 6" PX6 Double Dovetail Plate from Pro Media Gear is an Arca-type compatible plate that can be used as a rail or a plate for a long telephoto lens. Numbered scales on the sides of the plate allow you to make easily repeatable adjustments.
The double dovetail design lets you mount one dovetail to a clamp while attaching another clamp to the top dovetail. This is useful for macro and panoramic applications. A steel insert with a 1/4"-20 threaded hole provides added durability.