ABC stood for Advancing Blade Concept. The original prototype flew In July 1973 but suffered an accident due to a lack of available fore and aft cyclic pitch control. This was the first flight post the accident (which occurred in Aug 1973) and was the start of the definitive flight test program for the aircraft.

Source: "Sikorsky ABC flight trials" , FLIGHT International, 16 October 1975, Page 579