[the]

It's a fun little mystery!

I've never seen what I consider a definitive answer.

"Super Carrera" is a common answer, but I don't think correct. That was "invented" after the fact. It also doesn't really make any sense, given Porsche designations. "S" was always a designation, as was "Carrera." They have always been separate designations, with Carrera right above S. If the C stood for Carrera, the S would be redundant. It would be like calling an early 911S a 911ES.

My guess is the S of course is for "Super," the C is likely because (1) it was roughly the third of the series (small bumper, big bumper, big bumper with flares), and (2) it was remniscent of the 356SC, which was the last of the 356 line. By the mid 70s, the 924 and 928 were well under development, with the 911 expected to be phased out by the early 80s.