That's what he did in 2011, when Berkshire began buying IBM shares in March and didn't publicly reveal the purchases until November after it had accumulated a stake worth almost $11 billion.
The filing does reveal some minor Berkshire moves. It reports a new 20 million-share stake in Axalta Coating Systems. It's worth about $600 million at Friday's closing price. The Philadelphia-based company makes liquid and powder coatings for vehicles and industrial uses.
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The size of the stake indicates the stock was chosen by one of Berkshire's portfolio managers, not by Buffett himself.
Berkshire increased its Charter Communications stake by 42 percent to around 8.5 million shares worth $1.5 billion. Its holdings of Viacom fell by 32 percent, making the stake worth around $250 million.
Relatively small stakes in Phillips 66 and National Oilwell Varco were eliminated.
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There were no changes in Buffett's "big four" holdings: Wells Fargo, Coca-Cola, American Express and IBM.
In his live appearance on CNBC's "Squawk Box" on Monday, Buffett said he was happy IBM shares had dropped significantly since late July: "I love it when it goes down. It means the company buys stock cheap."
If he was doing any buying, however, we won't see it until mid-November, when Berkshire files its third quarter portfolio report.