"Super"
What exactly that was in a 1964 Pontiac, I don't know.
In earlier Hydra-Matics—which were a true four-speed—"Super" was basically just third gear. In early-'60s Pontiacs they used a variant of the Hydra-Matic transmission called the Roto Hydra-Matic, which was a three-speed with four ranges: "first" and "second" were differentiated solely by the torque multiplication changing as the fluid coupling between engine and transmission drained. Then the transmission would apply and release clutches like a "normal" automatic would to actually shift into "third" and "fourth". It was a weird transmission and only on sale for four years.