This is the rigid magnifier for the RB cameras. It has a couple of advantages compared to the more common waist-level finder. It is completely enclosed to eliminate stray light. It has a built-in diopter adjustment so that most people can use it without glasses. Finally, the eyepiece is larger so it is easier to see the whole screen.
The unit is in clean condition and works perfectly. It is missing the name plate on the front, but that is just cosmetic.