[morzh]

The pics #2 and 4 seem to suggest there is a hum-bucking coil there.

If you look at the #4 you will see how the left output of the transformer (seems to be a piece of copper mesh or wire that attaches to the transformer output's black wire) instead of going directly to the voice coil contact (the solder post) goes inside the field coil, and then returns to solder to the voice coil (the mentioned solder post's pin).

This is the hum bucking coil.

Actually the very last picture in the second post (pic #7) also shows it well.