A peek at the original schematic may provide the clues. At least during the GE production, the motors used in the E-15 and larger tractors have a very heavy field winding , that possesses few turns, in series with the armature. I'm not very up on my DC motor theory, but have dim recollections of this type of winding being used for either speed stabilization, or to cancel the effects of armature reaction.The placement of it will favor efficient armature rotation in one direction,and less efficiency in the opposite rotation.
One of the real EE types on this list will certainly clarify this. :)