V 4c slate p.12½ - State of the Art Forgery
A member bought this nice mint copy for a reasonable price from an overseas auction. On close inspection with a 30x lens, he discovered that the front of the stamp was replaced by a mint copy of the ordinary QV 4c. The back is from a genuine perf.12.5 stamp. The stamp was expertly regummed. However, looking under a good light source, the unclear "diffused-like" watermark caused by overlapping of the graft stamp is a dead give-away. Luckily, he got his money back from the auction house.
A large number of stamps have been "doctored" by the skilled forger in such a way to cheat collectors. So...buy it from a reliable source.