The mystery stock could be the highlight of the new 13-F report. Berkshire was a light buyer of stocks in the second quarter, purchasing nearly $4 billion, while selling about $7 billion, indicating it continued to pare its equity portfolio, based on information in the 10-Q for the period.

Warren Buffett’s Berkshire Hathaway revealed a position in embattled healthcare giant UnitedHealth Group while showing smaller stakes in longtime holdings Apple and Bank of America, according to a regulatory filing Thursday.