Every 13F on file since Q2 2013: what Buffett bought, added to, reduced and sold each quarter, largest moves first, with share counts adjusted for stock splits.
Each quarter is compared with the one before it. A quarter whose predecessor has no filing on file shows the book's size and no moves. Moves are ranked by the dollars that changed, at that quarter's reported prices; the twelve largest are kept.
Source: the manager's Form 13F-HR on SEC EDGAR, kept as immutable quarterly snapshots; changes compare two consecutive filings. A 13F is filed up to 45 days after quarter end and covers US-listed long positions only. Tickers come from a self-maintained CUSIP map; a position without one shows its CUSIP. Method notes in where the numbers come from.