BTW AM sound was always used with +ve vision modulation. I'm not sure
that
there was a killer reason why FM could not have been used with +ve vision
modulation, but intercarrier reception (the cheap'n'easy way to receive
FM
sound with TV) wouldn't work with +ve vision modulation unless there was
significant carrier amplitude remaining at the sync tips. Normally with
+ve
vision modulation the carrier amplitude at sync tips was nominally zero.

Early US TV sets used separate video and audio IFs -- intercarrier had not
been thought of at that point.

My understanding is that "inverted" polarity was used to minimize the
effects of noise bursts on the sync pulses.