Looks like some sort of improper repair. Maybe a spliced wire?

I've never seen that before

Oh! I think I know what that might be.

It looks like it's one of those rubber pad push button switches found on the control pads for various electronic devices.

For some reason, it looks like someone splice some under it then covered it in calk to hold it in place.

It could be a free play switch, if someone didn't know how to set the game to free play in the game's software.

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It appears to be something home made that plugs into the "electronic coin door" connector on the coin door interface board.

Coin doubler, maybe. Or RF control to add credits. (Like that mobile phone thing..)

Or NSA spy device.

But no, that's not standard on any WPC, -S, -95 game.