UK 6d air letter posted in Cape Town, South Africa, in July 1950. South African circular “T 4d” tax handstamp. Not complying with UPU rules this air letter was (i) taxed (less than) single deficiency only (4d UPU letter rate) and (ii) the amount due was not endorsed in UPU centimes. It even did not match the required postage due.
The air letter was diverted to surface mail, as the “By Air Mail” routing instruction was voided with red pencil. The UPU letter rate in July 1950 was 4½d. Hence, two times this amount should have been marked postage due.
The correct amount in UPU centimes would have been 90 UPU centimes because 1d = 10 UPU centimes at the time of posting. However, in the US (which had 3 UPU ctms = 1 cent) a postage due of only 10 cent was charged to the addressee.
Most likely, the UDS postal authorities did not recognize that the UK air letter was posted from South Africa and was marked postage due there. They mistook the taxation mark as a British one. The conversion rate between British currency and UPU centimes was 1d = 7 UPU centimes, yielding 4d = 28 UPU ctms, which was than converted into US cents at 3 UPU ctms = 1 cent (after rounding up).
