What does the "S" in 512 S stand for, anybody know?

S typically means Sport edition but the 512 is already that, so it may mean Special. Who knows. It often designates something beyond the base model.

 

It does stand for Sport. When the FIA limited Prototypes to 3 liters following the 1967 season, they also instituted a class for Sports Cars, which had a larger 5-liter limit but also required at least 50 to have been built, ostensibly for street use.

Initially, the only true race car to meet the requirements was the Ford GT40, as quite a few had been built as "customer cars" while the factory had been running the bigger 7-liter cars. John Wyer ran these successfully against the 3-liter Prototypes, winning Le Mans in both 1968 and 1969.

Considering the lack of competition in the class, the FIA dropped the production requirement to 25, and that prompted Porsche to develop the 917, which was still supposedly a "Sports Car" but we all know that it was effectively a Prototype. They dutifully built 25 cars to show the FIA, though at the time of the showing many were not actually complete, running cars yet. But it worked and the 917 was cleared to race. Several did appear in certain 1969 races, including Le Mans, but they had reliability and stability problems at first that kept them out of the winner's circle until year's end.

Ferrari picked up the gauntlet and then developed the 512 S to counter the 917s. I'm not sure they had actually completed 25 cars by the time the 1970 season began, but they did race right from the start of the season. So, like the 917, the 512 S was officially a "Sports Car", though they looked suspiciously like Prototypes to me!