Yamaha MU5 is a handheld synthesizer with a synth key type. It is a tone generator MIDI sound module that comes with a manual. The product is manufactured by Yamaha and is suitable for use in musical instruments and gear, pro audio equipment, and synthesizers.

Plastic on screen, ac adapter, manual.  Battery cover, coils rusty , doesn't affect performance and probably could be cleaned with contact cleaner(deoxit).