Not sure why this exists. Maybe an attempt to replace a lost lens cell, or achieve an alternate focal length. Marumi is a well known, mostly filters, optical company. Originating as Yorishima Optical but taking the Marumi name in 1952. I am not familiar with their lenses, but doubt if this is a 19mm lens. Doesn't seem to fit the cavity too well, does it even focus? The early pin release 11 0'clock lens should not be on a model III with slow speed dial. Maybe someone can advise about early Marumi lenses, as likely just a one-off experiment.