Although the sums involved are very small in terms of its annual turnover, the Royal Mail says the proceeds are used to recoup part of the cost of its National Return Letter Centre based in Belfast, which aims to return undeliverable items to the sender wherever possible.

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Daisywheel divides the alphabet into eight groups of four, assigning each letter to a controller face button based on the position of the left-analog stick.

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"If the purpose is to increase security checkpoint flow, there are much more effective steps we can take together to streamline the security checkpoints with risk-based screening mechanisms, " he wrote in a letter to Pistole.

CNN: March 10, 2013 -- Updated 0334 GMT (1134 HKT)