Wow as well.
If that is indeed genuine (as it appears to be) you seem to have something super rare in your hand. Not even Wikipedia has it in their Ace Tone product list (=>
https://en.wikipedia.org/wiki/Ace_Tone).
Perhaps even some kind of prototype?
I'd try and get in touch with Roland (the follow-up to Ace Tone, in a way) and see if they can shed some light on this.
In any case, don't sell it hastily. You might have a real gem and collector's item there, especially as it appears to be in good condition (at least optically; the electronics inside might need some (or even extensive) servicing after probably more than 45 years).