Six track was for the 70mm venues. (Where Star Wars was released first.) Then they needed the stereo mix for 35mm theaters as it opened on more screens, and then the mono because Dolby couldn't play properly on old mono gear at the time.
This was also in the days before audio mixers could "remember" all the settings, so other differences could happen in later remixing sessions.
It's a safe bet all the pre-1985 Laserdiscs have the 35mm stereo mix.