1943, 5¢ Poland, reverse printing. Of flag colors (bright red over black), o.g., never hinged, undoubtedly the finest known example of this Extremely Rare error that is listed but unpriced in Scott, a Superb gem; with 2008 P.S.E. Graded certificate (Superb 98). Scott No. 909c  Estimate $600 - 800. $0 (Image)