I would guess that the sound your talking about is the Dallas Rangemaster(treble booster),
these were designed to combat the drop off in high freqs. that a very long run of cable would cause, however the players of the day soon found that the gain could be set to way above just compensating for loss of treble resulting in some VERY famous guitar tones, early sabbath, zeppelin and probably loads more,
that would be my best guess.
i am on the very edge of geeking out so i'm going to stop here before i get into the science, i have done a lot of research into those particular tones recently,
S.