Campagnolo's Gran Sport is arguably one of the single most important rear dérailleurs in the evolution of the device itself. Lore had it for many years that it was the very first dérailleurs based around the now-standard "parallelogram" design - essentially, a pair of parallel arms swinging the lower body from side to side - as opposed to earlier whimsical spring or double-cable operated designs.
The originality of the design still remains questionable to this day. According to Frank Berto of The Dancing Chain:
"Most writers suggest that Campagnolo copied...the 1938 Nivex or the JIC. However, I believe the real inventor of Campagnolo's Gran Sport parallelogram rear dérailleur was probably Francesco Ghiggini. He obtained Italian patents on the design...[and] sold the rights to his patents to Campagnolo in 1951."
Origins aside, the Campagnolo Gran Sport went on to become the first successfully mass-produced parallelogram dérailleur, revolutionizing the entire industry, which set out to unabashedly copy the design and subsequent models.
The Gran Sport shown here is from 1961, an example of Gran Sport's fourth overall revision - largely cosmetic - from the original 1951 design. Despite these revisions, the geometry of the dérailleur remained essentially the same throughout its production; even the basic parallelogram was carried over to the later Record, Nuovo Record, and Super Record dérailleurs with relatively minor changes.
I leave you with a video of the 1961 Gran Sport demonstrating its operating abilities and characteristics. The limitations of the pulley cage should be apparent:
Gran Sport variants, courtesy Jon Fisher's Velobase.com
Gran Sport video, copyright Kurt Kaminer