Austria experimented with adding “varnish bars” to its stamps between 1899 (the first year of its new krone and heller units of currency) and 1908. These varnish bars were meant as a security feature to make it impossible for cancellations to be removed so stamps could be reused. They tended to compromise the design and printing of the stamp, however, and the idea of adding bars of varnish was abandoned.
Some collectors enjoy these oddities from Austria’s stamp past, and there are usually a few seen for sale at any given time. Take a look at the current selection; these varnish bar stamps tend to be unique. Varnish bars were added to several stamps, including those depicting Emperor Franz Joseph I.