Great Britain 1935 Silver Jubilee 2½d Prussian Blue, SG456a. An exceptional and very rare block of four of the unissued colour being the most iconic error of colour in GB philately. (Very lightly hinged on the top pair). An exceptional rarity and the first block of four of this famous stamp I have ever seen. SG Catalogue price for unmounted mint single stamps is £18,500. Accompanied by a certificate of authenticity from the British Philatelic Association (BPA).