The London Postal District was introduced in 1857/1858, with suffixes EC, WC, N, NE E, SE, S, SW, W, NW. Suffix NE merged into E in 1866. Suffix S split into SE and SW in 1868. Numbers were introduced into the London suffixes in 1917. Use of periods within suffixes ended with integration into the National Postcode System.
3L+4N 7-digit telphone numbers are not the same as 2L+5N 7-digit telephone numbers. The older 3L system required mnemonic exchange prefixes where all three letters were "meaningful" (e.g., ANDover 3423). The newer 2L system required only two "meaningful" letters; the third digit could be arbitrary.
Example: in Chicago, the format changed from 3L + 4N (e.g., ANDover 3423 [once the phone number of the Linotype Parts Company]) to 2L + 5N (e.g., AN3-3423). Mostly this didn't change anything at the time (AND is the same as AN3 on the phone dial); sometimes it did (e.g., Chicago exchange ABErdeen (previously dialled as 223) became ABerdeen 4 (dialled as 224)).
See http://phone.net46.net/ for historical details of conversions between telephone numbering systems (including tables with dates on which exchanges converted).
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