If Campagnolo had an embarrassing secret vice, it was its love for single pulley derailleurs. This despicable dalliance started with the 1952 Campagnolo Sport. It was still raging, unabated, certainly until 1968, when the Campagnolo Sport Extra was featured in the catalogue. It possibly even continued as late as 1973. Surely someone should have taken Tullio Campagnolo aside and told him to get a grip on himself.
These derailleurs pandered to an ancient fear amongst some cyclists that the extra friction of a second pulley wheel would hold them back. Everything that makes a good derailleur was sacrificed in order to avoid this dastardly friction.
In terms of gear changing (surely an important aspect of a derailleur) these models were about as much use as a chocolate fireguard. I once spent a whole afternoon trying to get a Campagnolo Sport to change reasonably across an undemanding mid range freewheel - to no avail. After this marathon session, it took only five minutes to remove it and fit and adjust a SunTour. Make no mistake, the single pulley derailleur deserved no place in the world after about 1948.
Various correspondents have commented that Italy had no obvious equivalent of the British Sturmey-Archer or German Fichtel & Sachs Torpedo three-speed hubs - and so humble shopping bikes were fitted with 3 speed derailleur systems with freewheels that typically had sprockets ranging from 16 to 22. Even single pulley derailleurs could handle these. To me this all seems very good, except that a basic, inexpensive, double pulley derailleur would have done the job better and a 16 to 22 freewheel is not that much use to anyone anyway. By way of a comparison, a plain-vanilla Sturmey-Archer AW hub gear would have been roughly equivalent to a more useful 16 to 28 freewheel.
Campagnolo developed three generations of these gruesome single pulley devices, all sporting similar ninja shaped pulley cages, but with different parallelograms:
I think that this is a relatively late example of a Campagnolo Sport, dating from or after 1956, as it has no cable adjuster.