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.
