Maybe Hilary Stone can help you. I just received the February Cycling Plus, and Hilary's Retro article this month is on the Campy Grand Sport rear derailleur, with attributions to list members Jan Heine and Chuck Schmidt. Maybe Jan and Chuck can help also.
I thought I generally knew about the older campy gran sports. I need help on this one. I figure it to be 1950's as it has the cable adjuster (That has a cast arm that causes it to set very far away from the body, different than all of my other G.S.) as compared to the cable stop. What is REALLY odd is the at the bolt for mounting it to the frame or in this case the hanger to mount it on the frame that does not have have a drop out hanger is aspring set up like the bottom end for piviting the pulleys. At first I thought someone had just been stupid and spaced out which side they assembled the spring on, but this has a very shallow coveron the back side also. That with the different castings tell me this is a different animal. Any Ideas? Thanks Craig Griffith San Luis Obispo,Ca.