Useful Date Indicators

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NOTE: These are just a few quick notes and links primarily for my own use in research.

1. US Zone/ZIP Codes

US numeric postal "zone codes" (e.g., "Philadelphia 3") were implemented beginning in 1943. They were replaced by ZIP codes beginning 1963. ZIP+4 codes were introduced in 1983.

2. London Postal Districts / English National Postcode System

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.

English National Postcode System introduced between 1959 and 1974.

3. North American Telephone 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 for historical details of conversions between telephone numbering systems (including tables with dates on which exchanges converted).

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