A St Helena 1868 block of 14 double surcharge and surcharge omitted errors will sell at Spink.
The lot is among the stars of the January 24 Philatelic Collector's Series Sale in London.
The block was almost split up in 1912, as an accompanying letter explains
The block is accompanied by a 1912 letter from a Charles Hasslacher, offering a single strip to a potential buyer.
Spink explains: “He suggests that he will split it if required as he has another gentleman who will purchase the second strip ‘as it makes no matter to me either way’ though he continues ‘from a philatelic point of view it is such a pity to break it’.
“The block has not been offered to the market since that time.”
St Helena is a remote island off the coast of South Africa that has been a British dependency since the early 19th century.
Napoleon Bonaparte was exiled there in 1815.
Its first stamps (including the present specimen) were overprinted 6d key type stamps displaying a profile of Victoria, which were used up until 1884.
The block is valued at £50,000-60,000 ($60,830-72,996).
A first printing Indian 1854 4 annas pair carries an identical estimate.
The stamp was among the first to be printed in two colours and was produced at the Survey of India Offices in Calcutta, under the orders of the British East India Company.
The issue remained in use for many years, long after the crown took the reins of the country in 1858.
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