Fortunately, this motor has 4 power posts.
On a series wound motor, the current goes through the stator field windings, then continues through the rotor (commutator) and then back to the battery to complete the circuit.
One of the power connections needs to be jumpered to another one to complete the circuit and the other two power connections go to the battery + and -. If you just reverse the battery polarity, it's reversing the fields on both part of the motor and it DOESN"T change the rotation.
You need to reverse the polarity of only just the rotor or just the stator. This was accomplished by running a jumper catty-corner compared to before to reverse the polarity of just the rotor.
Now it spins the correct direction!
Here is a photo of it jumpered correctly!
