Yea, on Presence it sounds like a flanger since each note starts a swirl sound, as opposed to a constant phase. Live, he kept things simple on stage, so prolly just didn't want to add another stomp box to the wooden pedal board he made. (Also, listen closely and hear the open B string ringing on the studio version, on the main track (not the octave higher overdubbed track). If you don't hear it at first, slow the speed down. This means he is playing the riff entirely on the G string. Just good to note, if performing the studio version live, and you wanna get that sound. But using an octave effect gets crapped sounding with that note added, and of course turn it off on the E chord at the end of the riff.) I'm playing it on the G string live (no octave effect tho), as well as now incorporating a 'shy slide' to get a more delta blues effect. All about the roots.. It sounds cool...