For example, the backing material may comprise any of the per se known flexible plastic support material heretofore used for such purposes, e.g. a cellulose ester such as cellulose acetate, cellulose triacetate, and the like; a polyester such as polyethylene terephthalate; or a polyolefin such as polyethylene or polypropylene, the polyolefins being particularly useful backing materials.