Bear in mind there is a (big) difference between being entered and being nominated. A publisher can fill out the paperwork, pay a fee and enter any author. But only a few of those entrants are nominated as finalists and actually considered for the prize. I believe if you read closely you might find that Bukowski was entered, but not nominated.
I don't say that based on any facts or specific knowledge that I have, but I think if it had been a nomination, it would have been prominently mentioned in every Black Sparrow bio, and in every magazine piece written on Bukowski from that time forward he would have been referred to as "Pulitzer Prize nominee Charles Bukowski." Both because it's a big deal to be nominated, and because Bukowski is not exactly traditional Pulitzer Prize material.
But there is no mention of it anywhere. If he was nominated, the virtual silence around the nomination seems very weird to me.