I suspect that grey box right before the anode cap might be a HV resistor. If so, there are a couple possible reasons for it, one could be to bleed off the anode voltage quickly and safely after the set it turned off. If that's the case the additional wire would probably go to ground. Another possibility is to detect whether the high voltage is present, and puts the set into some kind of protect mode if no voltage is detected. Sometimes its a combination of these two. You might check the neck board to see if there is an empty spade terminal, as it doesn't look like it could reach that many places on the main chassis, though certainly check any areas on the main chassis that it *can* reach for empty spade terminals.