It's so you can double up the rubber behind the drop targets. Install the first ring just as you normally would, then put the second one below it. It helps prevent stuck balls and busted out bulbs.

You can also chuck larger screws and bolts in your drill and polish them in a rag.

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Looking really good, but I think you might be soldering your lamp wires to the isolated lug and not the one connected to the tip of the bulb. You want to solder the wire from the lamp board to the "A" lug as seen in the diagram below. It looks like yours might be soldered to the "B" lug instead.

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As mentioned earlier, the Bally system is slightly different than what's described in the diagram. The bare wire is the 6.5v lamp bus, and the insulated wire goes to the lamp board whose SCR's give the bulb tip a path to ground.

You should not have continuity between ANY of the terminals on the socket. Do you by chance have a bulb installed? 44/47 bulbs have extremely low resistance and would probably read as continuity on most meters.

Lug B is completely isolated and is provided as a solder point for a diode used in a matrix based system.

Correct wiring is bare bus wire connected to C, insulated ground wire to A.