Great Britain 1840 1d Rainbow trial (State 1), SGDP20Great Britain 1840 1d Rainbow trial (State 1), SGDP20. A very fine and fresh four margin unused example in reddish brown. A very scarce State 1 example. 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 this assumption into question. It is believed that the wax