In this case I would think the Monroney label/ window sticker is maybe the only way to confirm if it was installed at the factory. The Monroney label was mandated specifically for the purpose of having the purchaser know exactly what they were paying for so the dealer doesn't keep or exclude an item or items. Items like wheel covers or floor mats might be in the trunk but other than the '68 Road Runner decals not originally installed I can't see how a major graphic like billboard decals would be left up to 100's of different dealers to install. If the billboard decals were subject to shipping damage then wouldn't the painted quarter panel and door also need extra shipping protection? Doubt if originally ordered they were installed in any other place but the factory. Could see a dealer adding them for a customer or as added dealer bling.