[Herald1360]

That extra wire doesn't sound like it should be there. I suspect that the extra volts are as surmised down to the lower Vf of the 1N4007s. Connecting C29+ back to the dead MR sounds like it's just loading the supply a bit and the voltage is being dropped in the series smoothing resistors. The fact that it's now about right is probably just fortuitous.

C29+ in the Trader circuit is the output end of the heater filter- I would be inclined to connect that stray wire via two more 1N4007s in series to C29- to give a better defined "zener" to clamp the filament supply at around 1.4 volts.

The filament supplies in these mains/battery sets with parallel filaments were always a bit "iffy"- a failed filament could cause a voltage rise which wouldn't do the remaining filaments any favours. Modern options like basic diode clamps or low voltage dc regulator circuits simply weren't available to the designers. Other designs used series connected filaments and powered the chain either via a dropper from the HT side or a 7.5V dc supply. Either way an open circuit filament simply stopped (or at least reduced, there were filament balancing resistors to bypass the cathode currents from the valves higher in the chain) the filament current with no risk to the remaining good filaments.

BTW MR2 isn't a bridge (4 diodes) it's a full wave rectifier or half bridge.