South Africa 1926 4d Triangle Privately Perforated on Letter

1926 4d grey-blue triangular, PRIVATELY PERFORATED, English (SG 33a var): 1933 Registered envelope from Rouxville (19 May) to Pretoria franked on reverse with the privately perforated 4d triangle and a 3d pictorial making up the correct 7d rate. A scarce illustration of the privately perforated triangle in use. VF..