I've noticed that on some cars the silver on the airbox is worn off or black alltogether and on some its pristine. Might have something to do with the cosmoline they coat the engine bay with. If your specific dealer, independent, previous owner, etc removed it with something harsh it could of taken the paint off with it. Don't really know why. Bet a bottle of brilliant silver touch-up paint could revitalize the lettering though...

Hope this helps...Lee