Complex sentence: On September 21, Brian completed the track after a final Electro-Theremin overdub was added by Tanner, later again in 1976 elaborating on the event: It was at Columbia.
Split into simpler sentences: On September 21, Brian completed the track after Tanner added a final Electro-Theremin overdub. In 1976 he elaborated on the event: It was at Columbia.