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Missus P is right again. In fact, usage of 'yr' for "your" dates from far before Kerouac. It is first recorded in 1772, in the journal of a man named "Knyveton" (I'm afraid the OED is not any more specific than that; anyone who can identify this man for me, please do) in the following sentence:

"The two rooms and the closet will furnish *yr. obdt. with lecture rooms and office."

Of course, use in a journal is not very official. Never fear! In 1811, it appears in a letter by Percy Bysshe Shelly, in the fragmental "Not that I like yr. heroine." 65 years later, statesman Benjamin Disraeli uses the contraction in addressing none other than famed politician and grammatical pedant, Winston Churchill. Quoth Disraeli, "I earnestly hope that these arrangements may be consistent with Yr Grace's decision to accept the high office of the Queen's Representative in Ireland."

It should, however, be noted that all of these are examples of very informal, private writing. It would be some time - till, indeed, the mid-late 20th century - that "yr" for your would make appearances in public.

Update 1/15/03: I have been informed by Cletus the Foetus that Ezra Pound included 'yr' in published poetry of his, which would in fact move the word's appearance in public to the early 20th century, not mid-late.