Great Britain 1929 10s 'Seahorse' Experimental plate proof. SG417varGreat Britain King George V 1929 10s 'Seahorse' Experimental plate proof, SG417var. Very fine huge margin imperforate example of the rolled impression with outer frame lines incomplete, printed in black on thick wove card at the Royal mint as an experiment into alternative methods of plate production. Between 1929 and 1934, the Royal Mint conducted a series of experiments and trials to improve the making of recess plates by borrowing some of the