First you need to understand that Campagnolo was very slow in adapting the slant parallelogram derailleur, even though the Suntour patent had expired 4 years earlier. One of the reasons was the European market was not very accepting of the Japanese components, which look was defined by the drop/slant parallelogram. The protuberance on the Croce rear derailleur was simply a sliding control arm (attached to an extension of the lower inside pivot) that opened up the parallelogram as it swung across the cogs of the freewheel, mimicking a slant parallelogram, but retaining the inline parallelogram look of the classic Campy derailleurs.