Electra-pianos
Beginning in 1970, RMI began producing the 300 Series Electra-piano, the sound of which became very familiar to fans of bands such as Genesis (who used the instrument extensively from 1974 to 1977), Yes and Deep Purple. The original 300A and B models had 61 keys. Eventually seven more keys were added to create the 368 model (but not touch-sensitivity).
The 368x was essentially a 368 housed in a new molded plastic case instead of the previously used vinyl covered plywood. In 1973, RMI came out with the Model 400 which was a 300 with a self-contained sound system aimed more at the home-user market, and the 600 series, which was a stereo version of the 300 with two tone generators per key, but also included some newer sounds.
Read more about this topic: Rocky Mount Instruments