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You are correct about prewar standards, but every prewar set has enough range in the horizontal hold control to work at 15,750 Hz. Occasionally a set won't have sufficient width, but that is usually not a problem.
Prewar CRTs were relatively dim, and had large beam size, so it is impossible to see the difference between 441 and 525 lines on them.
As for sound, it is amazing how well slope detection works. Simply detuning the fine tuning control about 100kHz puts the FM carrier on the slope of the IF, and results in detection. Volume is somewhat lower than with a FM detector, but is quite acceptable.
FM sound and 525 lines were adopted in 1941, and some prewar sets were retrofitted with FM detectors.
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