That shaft is cut and welded to a sprocket, the original shaft has a sprocket welded on, and there is a chain coupler that holds the two together.
I have one of these units in my '40. I know not to operate the unit in reverse in overdrive, but if the solenoid is off and one forgets to pull the lockout handle before reverse, what would happen? I'm assuming the unit would just freewheel and not damage anything but I'd like to be sure before I start driving with it.