DC polarity does not matter as wired from the factory. It is possible to re-wire the motor with a bridge rectifier so that that the motor will respond to DC polarity.
Thanks again. I getting a little smarter on this subject thanks to your helpfulness !! My understanding at the moment is that the post war open frame motors can run on both AC and DC equally well abet whistle/bell issues. If DC polarity is reversed, the motor will continue to run in the same directions since both the armature and field are reversed. If just the field or armature is reversed the motor will reverse. I would think adding a bridge rectifier would be the same as running on DC and would not reverse the motor. Modern engines use DC can motors. Adding a bridge rectifier to the DC can motors makes them work on AC. You can eliminate the bridge rectifier feeding DC can motors and run them directly off DC.