The transformer tap gives hum cancellation by drawing some out-of-phase current in the lower portion of the winding, something like a push-pull transformer would do. The extra section (red-brown) is only about 5-10% of the winding, and the screen resistor is in about the same proportion of the plate resistance of the 6BQ5. It was pretty common for SE outputs. Cost is only one transformer tap - the resistor and capacitor are there anyway...