Interesting that the poster refers to the 70mm six-track sound as “stereophonic.” Spielberg also referred to Lawrence of Arabia’s 70mm sound format as such when he recently talked about seeing it in late ‘62 or early ‘63 when he was 16.
As far as I know, Lawrence’s 70mm mix would’ve simply had five tracks behind the screen and one surround track. Star Wars’ 70mm mix would’ve been an early example of the “baby boom” configuration of Left, Center, Right, and Surround, with the left-center and right-center channels handling the bass. I guess you would call this 4.2.
By the time we get to Empire in 1980 I’m assuming the 70mm mix was in 5.1, ditto Jedi in ‘83, but I could be wrong.