Technically since the coil is an early slant pole, the red wire should be on the driver's side of the coil.
Didn't the slant coil have the polarity reversed from the later coil?
The wires were changed in November 1929 to register coil current when points were closed and the key was turned on.
The swap on an early car, if desired, should have been done inside the junction box by moving the black wire to the driver's side terminal as I remember.
Exactly correct and Benson wins the booby prize.

Sorry I don't have a booby to give you.
I did like Richard Wilson's answer though, as well as some others.
This winter I lifted the hood to remove a wood hood latch block to take a picture for Rusty. Right away I looked at the red wire on the wrong side of my slant pole coil and wondered how that happened. To confirm that the coil was wired backwards I switched my multimeter to the highest DC setting of 1000 volts, then touched the positive lead to a head nut and then touched the negative lead to the top of a spark plug with the engine running. The needle moved downscale instead of upscale, so that confirmed the wrong polarity.
Yesterday I finally switched the red and black wires to the correct sides. The coil output is the same, but when the hot spark plug center is negative, the plug fires easier. If my coil output was low and I floored the car, the plugs might missfire with the wrong coil polarity.
BTW, when Ford switched from the slant pole coil to the straight pole coil, they also switched the sides of the coil primary terminals + and -. However, there were a few slant pole coils made with the new switched polarity, and I have one of them. I may have been testing it when I switched the wires, and then forgot to switch the wires back when I reinstalled my original slant pole coil.
