Ed asks: "what is the point of grounding the P-lead if it is ground already? "

Answer: To keep it grounded.

A magneto has three main components, a dynamo, a high voltage transformer and a distributor. The high voltage transformer has a low number of turns primary and a high number of turns secondary. Both primary and secondary have one lead grounded. On the secondary the other lead goes to the distributor and then eventually to the spark plug(s). On the primary the other lead of  goes to the breaker points (a cam actuated normally closed switch). The other side of the points are connected to ground.

The P-Lead stud is connected to the point there the primary lead is connected to the points. Therefore, when the points are closed the resistance to ground will be zero ohms. When the points are open the resistance to ground will be around an ohm or two, which is the DC resistance of the primary winding.

In operation, the rotating magnet of the magneto produces an alternating magnetic field in the core of the transformer. With the points closed, the primary winding is shorted so a current is generated by the changing magnetic field. This current generates its own local magnetic field. When the points open, the current goes to zero and the local magnetic field collapses quickly. Since voltage is a function of the speed that a magnetic field changes and the number of turns exposed to the change, the rapidly collapsing field produces a high voltage in the secondary winding. When the voltage gets high enough a spark is established.

If the P-Lead stud is shorted then the primary circuit is never opened, the magnetic field never collapses quickly and the spark is not generated.

Disconnect the P-Lead from the stud on the magneto and verify your airplane's P-Lead is shorted to ground when the ignition is OFF and open when the ignition is ON. The P-Lead stud on the magneto will always be within a couple of ohms of ground.

Regards
Brent Regan