I think there might be a cam in there but it's hard to tell. The screw (in picture 3 and on the left of picture 5 and 6) seems to adust tension on the spring, but it doesn't "flip" to another setting. It just kind of adjusts in either direction and then always rocks back to it's original position, leaving the indicator (?) always pointing at the dot (picture 2). I have added a couple of more pictures.
http://www.dirtroadgallery.com/1977_...himano_600.htm