From what I recall from an interview I read, Geezer used worn out roundwounds not flatwounds. He didn't like the fact that his tech was putting fresh roundwounds on his bass every night. I think a big part of the 'Geezer' tone is the heavy right hand technique. Hitting the fretboard hard up over the last few frets, it is almost like he briefly performs hammer-ons, with each note being briefly preluded with a high 'wub' note. I've not slowed down a recording to check how often this happens, but that weird pillowy attack suggests this is the case.