I don't think that BB has anything to do with the Supercorsa frames per se, and doubt Cinelli had special bottom brackets for their own frames, though they could conceivably have modified them. Cinelli's first and perhaps most definitive design for its cast BB shell was the model SCM. My guess is that the S.C. refers to the model of the shell and not the bike to which it belongs.
(Sorry, had to edit since I goofed my first response.)