Correct. Those have been available for ages, but there's a mask ROM on the CPU that is undumpable, at least at this time. I have a huge collection of datasheets and I was unable to even figure out what the instruction set was.

It's likely to be something Japanese, and I know there's been plenty of obscure architectures from them over the years. OKI is known to have made the CPU for Casio, so have you ruled out OKI nX-8?

Scanning through my old datasheets I have one from the MSM65512 which probably means I considered that but it's been a while. I should have kept a devlog.

I found an old email to someone that suggests I thought it was an nX/4.

This is a lower-res version of the decapped chip.

https://upload.wikimedia.org/wikipedia/commons/4/44/Oki_m628...