Perhaps the authorities could be forgiven for not planning their postage due issues with the same care and attention lavished on postage stamps. After all, the best and perhaps only advert for your colony throughout the Empire was your postage stamps, one simply didn’t advertise that some people didn’t pay their postage charges in full and the best way of accounting for collecting those charges was to issue stamps just for that singular purpose.
Most colonies did indeed manage to have their ‘dues’ properly designed and printed by the same printers used for their main postage stamps but occasionally this didn’t happen and the resulting ‘primitive’ post dues are ever popular, including those produced by Malta, Fiji, and Palestine.
However, Zanzibar was unique in the fact that they had no country name inscription. The Government Printer produced the series of fourteen values (SG D1/17) inscribed “Insufficiently prepaid Postage due” between 1c and 75c on poor quality orange paper separated by a rouletted perforation, the 12c was also printed on green, the 18c on salmon, and the 25c on magenta papers of equally poor quality. The sheets were made up of ten stamps (2 x 5) un-gummed, and with sheet edges remaining imperforate. Certain values are extremely rare, the 6c is unknown in unused condition, and the first orange colours of the 12c and 25c priced by SG at £14,000 and £18,000 in unused condition!!
The early printings of the 3c to 75c values contained the error “cent.s” for “cents” at position R. 4/1. It was later corrected on printings of just the 3c, 9c and 15c. The 12c is also known with "I" of "Insufficiently" omitted which is a major rarity.
The stamps are known on covers of the period and are scarce and valuable. When used before about 1929, they were not cancelled, but later usages have circular date stamps applied.
In 1930 a similar, but larger postage due issue with fancy ornate borders, and also rouletted was produced, this is also extremely elusive found on mail.
The Sandafayre 5208 sale has a fine range of these popular issues including the errors and rare covers