"That was the first version of 1952. The 1951 versions are even rarer, and probably more expensive. The final, mass-produced version was introduced in 1953.
There was an article on all these early Campagnolo Gran Sport derailleurs in Vintage Bicycle Quarterly Vol. 2, No. 2."
I believe an in-depth check will actually show the production of the Gran Sport Extra (the model up for sale by Rocvale) to be far rarer than the preceding model(s) without the horn. Furthermore, as has been explained to me by many old-timers in Italy, Campagnolo's production was not 'production-year' based and the Gran Sport Extra was almost assuredly produced for less than a full year as Campagnolo had not yet started what would today be considered 'industrial production'and the derailleur was a work in progress. These same people have told me that many shops and mechanics put more Gran Sport derailleurs in stock when they first came out and did not sell them through too quickly so the short-lived Gran Sport Extra 'generation' was simply passed over in many shops.