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NAURU - 1916 10s indigo- blue Waterlow printing Seahorse lower-left corner block of four, each stamp with DOUBLE OVERPRINT, ONE ALBINO, SG 18a, never hinged mint, a few separated perfs within the lower sheet margin reinforced. The rarest stamp of Nauru, one of two known blocks. Each stamp with a very light 'BPMA' (British Philatelic Museum Archive) backstamp. Rare and beautiful. Cat £52,000 as individual hinged stamps.