[MAG]

The polarity on the coil is related to the polarity of the battery. On a Model A, if wired as original, the positive battery post is grounded to the chassis.
Constant voltage is applied to the (-) post of the coil.
The ignition coil should be wired so that the positive side (+ post) of the coil is going into the direction of the chassis, i.e. positive ground.
The wire going to the ignition switch from the coil (+) will ultimately find its way to the distributor. In the distributor, when the points close, ground is provided to the coil turning it ON.