Bicycle Mechanics - Shimano 600 FD Question

I have picked up a Shimno 600 front derailleur. Pictures are here:

http://www.dirtroadgallery.com/1977_Miyata_Liberty/Shimano_600/Shimano_600.htm

I have drawn some arrows pointing at a screw adjustment. What does that screw do?

Good question. I can't tell from the pics. Radial indicator like that one appears to have suggests that there is a cam involved, but I don't see one. Do you?

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 (http://www.dirtroadgallery.com/1977_Miyata_Liberty/Shimano_600/Shimano_600.htm)

It is supposed to adjust spring tension. If you turn it 180 degrees, it'll stay in place.

Thanks LWaB