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London had a harder time assigning exchange codes, because the city stuck with the 3-letter, 4-number format. (3L-4N was also used in the early days in some American cities, before they went to 2L-5N, including, I believe, Chicago).

Suitable letter combinations for London were becoming quite tricky to pick for some of the later exchanges, and the GPO sometimes resorted to adopting non-geographic names. They managed to use up a few prefixes by adopting poet's names; for example there was BYRon (297) and KEAts (532). London (and the other five British cities using the 3L-4N scheme) changed over to all-figure numbers in the late 1960s.