Great Britain 1840 1d Rainbow colour trial (State 2a), SGDP20aGreat Britain 1840 1d Rainbow colour trial (State 2a), SGDP20a. A very fine and fresh four margin example printed in dull rose red on white Wove paper dipped in prussiate of potash. A most attractive piece. 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 this assumption into question. It is believed that the wax could not have withstood the