I'm a little late to the party, but this is how it goes. The insulation between the commutator bars determines if it should be undercut or not. If the insulation is Bakelite, DO NOT undercut. If it is insulated with Mica, DO UNDERCUT. Undercutting removes a little of the insulating material between the commutator bars. The armature on the left looks to be for a low torque starter and the one on the right looks to be a high torque armature. Count the commutator bars. 23 bars is high torque and 27 bars is low torque. The high torque armature is wider than a low torque one. The high T armature won't fit in a low T case, but the low T armature will fit in a high T case, but even though it may spin it won't have any power.
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