I do not know if this is the answer, but old-time typewriters did not have a "1" key. If one wanted to type a number with a "1" in it, one had to use the "l". Your question seems the backwards of the old typewriters.
Emmett
2013/05/10 9:09am
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If you're talking about its use in unsolicited E-mail, I'm guessing it's just a way to avoid spam filters.