I have seen plenty of "Heavy Metal Meltdown" pinball wooden pieces on games like Xenophobe, similar era games, but this was a first. I can't believe there were any extra Addams pinball cabinets, unless they were mistakes...
This usually happened when there was an error in the screen printing. I have a game, pacland I believe...that has mspac on the bottom. It only has one or two of the colors screened. To me it would have made more sense to screen the other side and make use it for the opposite side.
Its not all that uncommon to see this. I have seen sideart on the bottoms of several midway cabs.