From the owners manual:
“The MP-1 is an epoch-making keyboard instrument. It is the first ever musical instrument to have a built-in music score printer, and it also has a two-channel memory function. With these revolutionary features, you can increase your enjoyment and understanding of music. The MP-1 has many other exciting features, such as a variety of instrument voices and rhythms, easy-play Auto Bass Chord system, and the completely new Duet function.”

see also:
Yamaha MP-1 (1982) First and only keyboard with score plotter?
Forum dialog at
Service Manual (watch for ad links)