What is a 'Special Six'?

Well the HyperPak was '60-'61 only and these are '68-'70 engine codes. So It can't be the HyperPak. (And my Chilton's is a '71 so I don't have info if these codes go beyond '70.) I went through my early 80's Driect Connection engine book (man, that is one awesome book; GET one if you ever can), and I can't find any reference to this. They mention the HyperPak and the Super Six and the Lite/Feather Duster engines, but nothing else I can find.

So perhaps this was indeed something for trucks or taxi's etc. Another of life's mysteries.....well, thought I would ask. If anyone comes up with anything solid, I would like to know.