In 2013 I started sending out mail art that was intended for the recipients to hide the art in a book. A new set of ten pieces going out today. Paintings of bookcases on unused franked postcards form the 1980s. I needed to augment the original postage with additional stamps. In this case some even older unused stamps. If you receive a piece and hide it, years from now it will be a bit confusing to the finder as it will have postage from different decades.