Short answers, pulled from the story.
The initial intent behind the S-V was to support U.S. military launches throughout the decade. Engineers designed this third stage to use liquid hydrogen and liquid oxygen as propellants. The design aimed to boost lift capacity for U.S. military operations in the early 1960s.
NASA eliminated the requirement for all Saturn I launches to carry the S-V stage in May 1961. They switched to a two-stage version instead, leaving the military needs to other launch vehicles. Convair delivered two stages by February 1961 to meet these demands before the change.
Four specific flight missions occurred between 1961 and 1964 using water ballast. Each test flight reached suborbital altitudes rather than entering orbit. The first flight took place on the 27th of October 1961 at 15:06:04 UTC with an apogee of 136.5 km.
Scientists released water into space during apogee on select test flights for Project Highwater. This experiment aimed to study the behavior of liquid clouds in the vacuum of space. The SA-2 mission released 86,000 kg of water at its peak altitude.
Modern derivatives of the stage are still flown today on the Atlas V and Vulcan Centaur launch vehicles. A version of this stage was used on the Atlas-LV3C as the Centaur upper stage. This makes it the only Saturn rocket stage currently operating in spaceflight history.