E12 stamps available on the first day of issue were from four printing plates: 13916, 13917, 13918, and 13920. Plate number singles from plates 13916, 13918, and 13920 are found on Worden covers. I have not seen the same from plate 13917. This page shows plate number singles 13916 and 13918. An earlier page showed plate number 13920 on a Worden cover.