In 1976 Electra MPC (Modular Powered Circuits) models featured a pair
of cartridge slots in the guitar body, which allowed effect modules to
be plugged in and controlled from the front of the guitar. Today the
unusual thing is that the effects are onboard, but even offering
electronic effects to consumer musicians was fairly new at the time and
offers an interesting alternate way to do it. There were a total of 18
guitar models which carried MPC circuits. The most notable was the