1917-19 Flat Plate Printing - no watermark (#498-518)
THE EXPERIMENT CONTINUES While Perf. 12 made stamp sheets and coil rolls too brittle, the adoption of Perf. 10 caused the opposite problem -- Perf. 10 stamps were difficult to separate, often resulting in torn, ragged perforations. The Post Office decided to try Perf. 11 as a compromise.
