Points closed equals a hot circuit with magnetism being generated in the coil core. It's going to drop voltage due to the electro-magnetic effect with resistance in the primary winding. The coil can get hot if it stays energized too long. If the points are open then there is no ground path due to the open circuit.
A coil has a single internal connection between the primary and the secondary coil windings. This connection has an effect on efficiency of the electro-magnetic effect. With a Positive ground system, the power is always on the negative side for battery and coil. The ground side of the coil is the positive terminal that is connected through the breaker points & condenser to ground. It is opposite of a Negative ground system. With so many different manufacturers of coils, its hard to say if the secondary connection is at the positive terminal or the negative terminal. This is where the coil polarity tester comes in handy but a graphite core pencil can also be used as a tester if you follow a procedure that will give a proper indication.
THis is a good link.
https://mgaguru.com/mgtech/ignition/ig104.htm