Remove the slate+white wire from the J relay. Leave the wire that goes from this lug to the coil in place. That wire "seals in" the relay. In this game, that wire was brown but YMMV. In place of the slate+white wire, solder a new wire that is about 12 inches long. Note: the other wire on this switch blade pair is brown-white-red but Gottlieb reds fade badly so this can't be seen in the photo.

Next, add about 9 more inches of wire to the end of the slate-white wire that was removed.

Add a normally closed switch to the end of the P relay. Solder the extended slate+white wire to one lug. Solder the new wire that you added from the J relay to the other lug. Which wire you solder to which lug is immaterial. Here I used a white wire with an orange tracer.

The switch that is added should be arranged as such: spacer, long blade, spacer, spacer, short blade, spacer, switch stack backing plate, screws. Using the right length screws is important as they cannot impinge on the relay armature.

Note the heat shrink tubing where the slate+white wire was joined to the added white+orange wire. Zip-ties were used to tidy up the wiring.

Note the dotted box just to the right of the J relay coil symbol. The box encloses the new normally closed switch we've added.

This schematic diagram was originally provided by Scott Charles. I've moved the location of the NC switch to reflect where I implemented it in this game.