[Mondial]

The black wires are a negative feedback winding which was added in later production. It applies an out of phase IF signal to the suppressor grid of the 6K7 tube and serves to stabilize the IF amp. I guess Philco had problems with the IF stage oscillating and came up with the extra winding as a fix. Don't think it has anything to do with your problem unless it is obviously shorted to the secondary winding itself.

It seems that the secondary of the IF trans is not resonating with the associated trimmer and that will definitely cause low sensitivity. Have you measured the actual resistance of the secondary and how does it compare to the value noted on the schematic?

There is not much there to the tuned circuit, so there is a problem with either the coil, or the mica compression trimmer which tunes it to resonance. You can disconnect the coil winding from the trimmer and test each individually.