The middle derailleur in the picture is a mysterious model. I guess that it might be the legendary (mythical?) Sanko Procyon PV-II. Some sources claim that this has plastic parallelogram plates - although, from what little you can see, they look like metal pressings.

• Publisher: New Cycling
• Country: Japan
• Date: September 1966
• Derailleur brands: Sanko
• Derailleurs: Sanko Procyon PV-I, Sanko Procyon PV-II?, Sanko PA, Sanko PF