I going to assume that it's a moog synthesizer, maybe it's a mellotron. What's interesting though, is that I opened this live performance to play in the background and randomly noticed what Keith was doing to that poor instrument.

https://www.youtube.com/watch?v=5u4HgJatKOY&t=891s (14:51) he jumped behind it and started tickling moogs innards, which resulted in a terrifying sound.

https://www.youtube.com/watch?v=5u4HgJatKOY&t=1182s (19:42) is just violent... Keith jumped on it's keyboard and started rocking it.

Jokes aside. ELP lives are quite interesting. There are these strange moments, improvisations, Lake might slide in a quatrain from Epitaph. In Karn Evil 9 there is a computer with wings.