The single wire GM SI Series alternator may not be self exciting, so it will not produce current to run the Model "A" electrical system. Inside of the alternator there is a stator, and it must have some residual magnetism to start the alternator. Revving the engine may provide the starting magnetism, but there is a way to make the SI alternator start immediately.
Watch this video and pay attention to his discussion of the SI Series alternator.
https://www.youtube.com/watch?v=QPl7CTb685I