The Nikon SR-2 ring flash was introduced in 1975 and discontinued in 1989. Its main target is for close up photography. The flash is powered by a power unit or by a portable battery holder. It connects with one cord to the power unit and a sync cord to the camera.
The SR-2 has no automatic method to meter the flash light. It is all manual. Supplied with the flash is an exposure calculation chart.
The ring flash mounts to the lens 52 mm filter thread. So you can use all lenses that have a 52 mm thread. In this case it is mounted on a Micro-Nikkor 55mm lens.