Great Britain 1841 1d Rainbow Trial (State 3), SGDP20Great Britain 1841 1d Rainbow Trial (State 3), SGDP20. A superb unused four margin top marginal example printed on white wove paper in deep green. A scarce specialised philatelic item of early GB postal history. These trials were produced to test various combinations of stamp and cancellation ink. It has long been considered that the voiding of the corners was achieved by filling the grooves of the plate with wax. However, recent research has brought