DDR - official stamps B (Verw. Post A / Zentr.) 1956

GDR - official stamps B (administrative post A / centr.) 1956 Catalog no. 3 XII 20 Pf. cipher in the square with watermark 2 XII as postage due single postage paid on window cover from 'Berlin 21.4.56' with arrival cancel of Erfurt. Photo certificate Jörg Schönherr BPP.Covers of this watermark type not listed. Extremely rare variant for the official stamp collection.