That’s what he originally had. ...Then he had a rough patch and asked his investors to switch to 2/20, so that he could keep operations running also during bad years. His investors agreed and 2/20 was born...
He never had a rough patch running the Buffett partnerships, or changed structure. He made money every year and his returns were roughly 40% annualized. Source: I’ve read the partnership letters.
And I’m pretty sure 2/20 was already a common fee structure on Wall Street before Buffett started his partnership.