The Casio DG-20 was an "entirely new type of digital musical instrument which features the latest in state-of-the-art technology". Played with untuned nylon strings, the notes on the fretboard correspond to that of a guitar in standard tuning so it's a breeze to play. These are the MIDI functional models! 
Tested and working on a power supply, the battery compartment has some corrosion so likely won't work on batteries. The pads and strings work well but are a bit old so it needs a deliberate playing style for the sensors to function perfectly.