How old is could of, should of, would of, the controversial issue reported on elsewhere in this blog?
On reading the proofs for my chapter in the second edition of The Oxford History of English, edited by Lynda Mugglestone and first published in 2006, I was reminded of the fact that the writer Betsy Sheridan (1758-1837) had written in her diary on 16 September 1785: “I should not of known her” (ed. Lefanu 1986:69). As early as that!
Bridging the Unbridgeable 
I’m impressed! Normally I see should of and related constructions in very casual contexts, such as blog comments and Twitter, but it does appear in edited writing too: Cormac McCarthy uses it in his novels, perhaps to evoke his characters’ accents or likely spelling. I never considered how old it might be.