This extra hole is present on all wooden Nintendo control panels, including the cabarets. It was used for Sky Skipper.
The KLOV listing for the game lists it as one button, but look carefully at the flyer images and you will see this extra button hole populated on both the upright and cabarets. On cocktails the extra button was stuck on the side of the control panel (easiest to see on P1, in orange). Watching video of the game, I'll be damned if I can figure out what the second button is for, but it's there on the flyer.
http://flyers.arcade-museum.com/?page=flyer&db=videodb&id=2572&image=2
http://flyers.arcade-museum.com/?page=flyer&db=videodb&id=2572&image=1