20 vertical pair in a block of four! Margin or corner margin pieces, unperforated in the center! Genuine Cyrillic postmark "PSKOW -k- -2 1 42"! In used condition, rarely seen! On the upper left stamp with plate flaw I, a great rarity of the area, unsigned! Photo certificate Walter Zirath BPP (2025): " The condition is flawless. Only a few pairs are known.", Michel -.-, as it is practically never offered!