This type of viewfinder was developed by the J.A. Maurer company for use with their 70mm cameras during the Gemini program. The viewfinder attached to the mounting bracket on top of the camera, its the lens could be flipped up. The concave lens provides an illuminated outline to pictures being taken, which is particularly helpful in dark conditions.
NASA transferred this object to the Museum in 1971.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.