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@clubman
OK, so we know which way up the jockey cage goes, now here's a photo which shows the main difference between the Tour de France (shown here), and the Juy 51 shown above.
I think it's probable that the reason the attachment of the plunger to the jockey cage was moved to the middle of the cage for the Juy 51 was to make it easier to get onto the bottom sprockets. If the cage is slightly further away from the sprockets there's a bit more space.
I note that it is claimed that the Juy 51 can take a 26 t bottom, by an odd coincidence the same as the Campag!
It's worth remembering that a plunger derailleur moves parallel to the wheel spindle, whereas a parallelogram changer moves down as it goes towards the bottom sprocket.
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