South Africa 1933 1d PO Museum "Proof" Imperf Frames, Rare

1933-49 1d grey & carmine IMPERF BLOCK OF FOUR, FRAMES ONLY (SG 56var). Right marginal, superb UM (mount in margin) upright watermark. Very rare - only 24 pairs existing outside PO/Cultural museum. Spectacular exhibition piece. See additional image below.