The lettering on the plug strongly hints that inside the plastic there is a

0.47 MF capacitor rated 250 volts.
Very common.

I would suspect that badly designed plug vibrated up and down
and over time a fatigue crack worked its way across the copper/cadmium plated mounting tab.
Tab could be plated steel.

Since the part is no longer available I would try to solder a wire a few feet long to what is left of the tab in the plastic plug and then ground the other end of the wire to a vehicle ground off the engine that is easy to see and inspect, and where vibration is much less.

The other bank of the engine should also have a capacitor that does the same RFI job but looks and is mounted differently.

In addition to reducing radio interference, the capacitor also reduces the “spike voltage” on the ignition coils
so in the worse case the way more expensive coil(s) might fail in the future.

Or if the plug is still available, buy a capacitor and attach one lead to the plug and the other lead to a soldered or crimped ring terminal.

I would bet that a lot of those have factory RFI reducing and coil voltage spike limiting parts are now cracked.