This circuit might be the long-sought "Harmonic Balancer" invented by Western Electric. Although it looks at first blush like feedback, it operates quite differently than the usual overall loop feedback. For one thing, it doesn't do anything at all if the circuit is perfectly balanced ... the AC currents flowing between the center-taps of the two transformers will be zero. It only acts on unbalance currents that flow across the PP stage. In other words, the feedback is fully effective if one tube is pulled out of the circuit, and in practice helps to balance the PP pair of tubes under dynamic conditions. As with the neutralization technique described above, you need to make careful measurements under different drive levels, including deep clipping, to determine the right ratios of capacitors and the voltage divider.