Thinking more about it, the name OPsix makes total sense. It’s not really an FM synth at all, or at least, not entirely. It’s a polyphonic operator bank with 4 different synthesis models.

1)Classic FM
2)Ring Modulation
3)Wave Folding
4)Filter FM

A lot of people think about ring mod and folding as an effect, but this is going to be completely different. These are like komplex oscillators to the nth power.

I’m a bit confused about the wave folding model though, as the diagram doesn’t specify modulator and carrier, unlike the other 3 formulas, just ‘input/output’. Maybe someone who understands wavefolding better can chime in.