Great Britain 1840 1d Rainbow trial (State 3), SGDP20dGreat Britain 1840 1d Rainbow trial (20 November 1840), State 3, SGDP20d. A very fine imperforate four margin bottom marginal example printed in drab on thick bluish laid paper. Showing "Tiny dash" in SE corner square from positional R. 3 2. 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