There seems to be a desriable effect, possibly a mechanical derived feedback, from leaving the windings partially free to move. At least in most of the vintage iron, this seems to be the case. If a conductor moves thru a magnetic field, there will be a voltage induced. So if a transformer winding moves slightly at an upper frequency, there might be enough displacement to cause a slight voltage shift in the reverse polarity of the main voltage induced in the winding. I am no expert on this, but am researching it as time permits.