Yamaha FM not FM? CASIO CZ-1 Phase Distortion Synthesizer

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Phase Distortion synthesis was Casio's implementation of Yamaha's FM synthesis.  Or was it?

Here's something interesting I found looking up Phase Distortion synthesis on Wikipedia (anyone confirm|deny this?):

"Phase distortion synthesis is a synthesis method introduced in 1984 by Casio in its CZ range of synthesizers, and similar to phase modulation synthesis in the sense that both methods dynamically change the harmonic content of a carrier waveform by application of another waveform (modulator) in the time domain. Casio introduced the term 'phase distortion'.

Yamaha had previously produced the first Phase Modulation synthesizers in their DX series of synthesizers: Yamaha mistakenly promoted their machines as FM when in fact used phase modulation. Both methods produce composite waveforms whose harmonics are the sum and difference of the carrier and modulator waveforms"

Click the pics for the larger shots.  I never noticed the display was recessed before.  Also never noticed the grid stripes on the side panels.  Finally, lots of zeros in that serial number, SN B00000980.