This is a camera ad printing block from after 1954 when Keystone split for the last time. The Keystone Camera Company was formed to legitimize their equipment as quality products and shake the kids toys reputation. The plate is 10 inches long.
From my private collection.
some are made of gray cardboard and some are made of lead. The lead ones are called Zincs by printers. Generally linotype lead was poured unto molds. I worked for a newspaper when these were used.