The 600 series combined the sounds of the Electra-piano and the Rock-Si-Chord. The first model was the 600A (1968). It included 8' and 4' tabs, which required two oscillators for every key and a separate bank of filters for each footage. The 600B (1970) featured an improved voicing circuitry and a pedal board with two volume and two sustain pedals. The 668 and 668X were the equivalents to the 368 and 368X respectively. The 668X weighed and cost $2,095. It was the last analog Electra-piano remaining in production in the early 1980s.