Looks like a DPST switch. Typically it would have been done with a SPST switch and one pair wire instead of two pair. But it should work ok. It should have a cover for safety. How about this for a cover?

https://pinside.com/pinball/forum/topic/worst-hackrepair-you-ever-saw/page/53#post-5614776

I second Mopar's method tieing into the bounce switch, as this is how the game was originally designed to switch off. Also agree with his recommendation/link to the push button SPST switch or equivalent. I like push button rather than toggle design, as you can't see the toggle under the cabinet and sometimes you have to feel around to operate it, but with a push button all you have to do is find the button and push. If you want to make things safer, go with the 3 prong grounded plug and ground the cabinet metal parts.

True. But why would you want to touch a live wire in the first place?

On the other hand, if a live wire happened to touch anything ungrounded in the machine (side rails, legs, coin door, etc...) and you happened to touch any of those ungrounded pieces of metal, you would get quite a jolt. This would be an instance where you were not working on the machine, but rather just playing it with a defective wiring condition.

Did I miss something? Is there a reason not to run a 3 wire, grounded system?

Add the switch in this line shown in the attachment. Leave the bounce switch in the circuit...just in case someone wants to bounce the game!

Screen Shot 2020-04-28 at 9.27.37 PM (resized).png

Yes

Good question. I think they should also be grounded, because if the button were to become energized and you had your finger touching the button and hand on the grounded side rail (i.e., playing the game), then the voltage would be conducted through your flipper button finger to palm on the side rail and you would get shocked.