Great Britain 1851 2d violet blue Plate 4, SG15aaGreat Britain 1851 2d violet blue Plate 4, SG15aa. A superb used four margin example lettered 'JA' on thicker lavender tinted paper, with part sheet margin and inscription at left and large part adjoining stamp at right. Neatly cancelled by a Derry "172" numeral. An exceptional example of this rare colour shade. Accompanied with a 2006 David Brandon certificate of authenticity. In 1851 a few sheets of the new Twopenny Blue were printed and something