For someone who wants to closely examine the motor in a GP7 here are the armature and field configurations. A 1951 would have the earliest field. Note the only visible difference between this and the second armature is the position of the commutator slot relative to the gap in the laminations closest to the commutator. The other difference is random vs layered windings. However the fields are different. The earliest field is 17/32 while the second field is 20/32.
The third design armatures repositioned the commutator in relation to the bearing. The field was also redesigned with the square shoulders at the top. This would likely be in 5 digit engines.
