Leo didn't like the deadzone in the middle of an alnico pickup, he felt that eliminating the deadzone would make the pickup more efficient at generating signal. So he bottom-loaded the magnet and the polepieces are one magnetic polarity all the way through the coil. The alnico poles are north at one end and south at the other, so in the middle there's some section that's neither north or south.
MFD polepieces are made of two pieces: an outer, fixed part that touches the magnet at the bottom, and an inner, threaded part that screws up and down to balance string volume. Both parts are magnetically active.