South West Africa 1926 Squared Circled Windhuk Mark, 3d Pair

1926 Registered envelope (230x100mm) from Windhoek (5 MAR) to Cape Town franked with marginal pair KGV 3d blue (Setting VIa, SG 32var / CC55). WINDHUK squared circle Registration handstamp at left, very rare. Fresh letter, elusive franking.