In 1977, German GSG9 personnel stormed a Boeing 737 that was on the ground in Somalia and being held hostage by Palestinian (?) terrorists. All hostages were rescued. Later, in 1978, French Foreign Legion paratroopers along with some Belgians jumped (from around 800 feet) into Kolwezi, Zaire to free European civilians being terrorized by local rebels, again successfully. So there had been a couple successful operations before then. Although I suppose the Foreign Legion operation wasn't technically an SF operation.