If I remember correctly Keith recorded the iconic riff on an acoustic into a tape recorder. By turning the volume up they could make the tape deck distort. Other instruments were layered over this. Between Wyman's homemade short scale fretless and Keith's tape deck (which was an early portable reel-to-reel), I'd imagine there was plenty of slop. They probably just used their ears and did whatever sounded good. That's rock 'n' roll. Like Carl Perkins going to a C on a blues in E. Nobody ever told him the rules so he played it his own way.