I own a defective Arton IK-51 music keyboard prototype made in USSR by Formanta (creators of the mythical Polivoks synth), which was their only instrument with digital sound, accompaniment and midi. The hardware contains 3 large PCBs (crowded like early 1980th arcade stuff). One of the eproms is corrupted, because they lost their stickers and so likely ate light. So it crashes after few key presses and the accompaniment is garbled.
I also own the Vermona SK-86 keyboard made in GDR, which contains Z80-based hardware with many logic ICs and analogue percussion. I dumped the eproms and even bought the original ultra-rare service manual of it, which (unlike the many Casio and Yamaha home keyboards) looks detailed enough to emulate the whole thing.
And I own a Weimar mantle clock made in GDR, which contains a bizarre monophonic digital beeping squarewave chime circuit made from many logic ICs and one eprom. One of the melodies (selectable by DIP-switch) may be a fragment of "Ein Jäger aus Kurpfalz). This also may have been a prototype, because I never saw another clock with this sound circuit nor found anything online about it.