VERY FINE-EXTREMELY FINE. A BEAUTIFUL COMPLETE SET OF MINT NEVER-HINGED CORNER-MARGIN EXAMPLES OF THE PAN-AMERICAN ISSUE, WITH MIRROR-IMAGE NUMERALS PRINTED IN THE SELVAGE.
To insure that the correct vignette and frame plates were used together, the siderographer entered the denomination on the upper right and lower left corners of each frame and vignette plate. They were engraved to be right-reading for the benefit of the pressmen. On the printed stamps, these markings appear as a mirror image and become visible in the upper left and lower right corners of the sheets. The lower right markings, shown in this set, are also inverted, so that the pressmen would always see the markings right side up, even if he was holding the plate upside down. On some of the earlier plates, small "sequence" numbers were added as vignette or frame plates were produced. These small numbers can be observed on the 1c and 2c values here.
Scott Retail with no premium for the positions (why these significant markings are ignored in the catalogue is a mystery to us)