According to @KobeissiLetter, Berkshire Hathaway spent two quarters accumulating a mystery stock and has now disclosed a $5 billion position in UnitedHealth (UNH). According to @KobeissiLetter, the position confirms that the mystery stock is UnitedHealth, the largest U.S. insurance company. According to @KobeissiLetter, the disclosure centers on stake size and company identification, with no additional trading details provided by the source.