This is a Multi-Purpose Rail. Really Right Stuff makes Multi-Purpose Rails in a variety of lengths, from 73mm to 192mm (2.4-inches to 7.6-inches).

1. Use as a lens plate for a collared lens.
2. Use as a camera body plate for some medium and large format cameras.
3. Add a clamp to the rail and use use as a nodal slide for panorama photography.
4. Attach to the bottom of any object to mount in a Really Right Stuff clamp.
5. As of May 2012 this rail replaces the legacy B97 lens plate.

Specs:

1. Length = 2.8 in / 71 mm
2. Width = 1.6 in / 41 mm
3. Weight = 1.4 oz / 40 g