This phrase, closely kin to “would of”, shows up in a line of dialogue early in McCarthy’s No Country for Old Men.
Unlike Harrison, McCarthy uses eye-dialect (and has the tic of omitting apostrophes in contractions—where did this come from?), but it’s inapplicable, strictly speaking, in this case.
The google books corpus reveals a similar usage on p. 247 of Gass’s Omensetter’s Luck.
Page 72 has, in dialogue, “I wouldnt of thought it.”