What is a 'Special Six'?

I have read about the Super Six 2 BBL from the late 70's. But I was browsing through my old '71 Chilton's manual and checked the engine option codes for the Dart/Valiant. In '68-'69 there was a 'Special Order 6' in addition to the 170 and 225, and in '70 there was a 'Speical 6'.

So what is a 'Special 6'?