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The manufacturer may have decided that resistors were cheaper than coils, and it being a series string they probably had excess string voltage to burn off. The resistors were probably chosen to act like decoupling coils (resistance = coil reactance at the IF frequency) to avoid unwanted feedback in the IF strip. They'd then reduce the ohms of the dropping resistor between the high side of the powerline to the top tube of the heater string.
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