Transatlantic cover to Boston
August 29, 1851, folded letter from London to Boston posted at the Lombard Street post office in London. Letter marked in London for 19-cents debit to the US. It was carried by Cunard steamship "Niagara" from Liverpool to New York, where it was marked for 24-cents postage due, the convention rate for an unpaid letter up to .5 ounce. Since a British contract mail steamer carried the letter across the Atlantic, the UK was entitled to 3-cents inland fee and 16-cents sea postage under the convention. The US retained 5 cents.