The optical viewfinder can be slid into the 'hot shoe', where the flash is usually also inserted.
The viewfinder is very compact and offers a transparent and clear image. Can be used on various rangefinder cameras such as Leica M, Leica screw-mount, Leixa X, Ricoh GR, Voigtlander RF ...
The viewfinder lens is an optical resin lens.
(Camera on the picture is only illustrative.)