The genuine Cosina/Voigtlander diopter correction lenses will fit the following cameras:
Bessa R2, R3, R4 and Bessaflex single lens reflex.
Fujifilm X-Pro 1
Nikon FA, FM, FM-2, FE and FE-2
Zeiss Ikon 35 and some Contax cameras.
They come in various strengths to suit your eyesight. You can check with your optomotrist to find the correction needed.
There is also a replacement unit complete with rubber ring should you lose your original. Select from the drop down list.