Plate modulation is not a way of producing SSB. The SSB signal must be
amplified linearly, and generated at low level. FM, I believe, not only
can but benefits from being run through a Class C stage. The idea with
plate modulation was that the RF path was CW all the way to the finals,
and the audio directly modulated the final amp. The modulated signal
then had to be handled only in a linear fashion, but that was at the rx
end-the last stage in the tx being, obviously, the final.

The gizmo we are looking at seems to be a simple mod xfmr placed in
the plate supply to the 6146s in the tx. An audio amp such as a stereo
or guitar amp hooks to it and the mic goes to a preamp. In fact my
guess is back then a guitar amp was exactly what was most used.