If institutions were piling into shares of Berkshire earlier this year, they would have had good reason. Buffett has set a floor on the stock through his recent addition of a share buyback program, which he set at the level of Berkshire Hathaway shares falling below 1.2 times book value.
"That provides a margin of safety," Koenig said.
Berkshire shares traded very close to that limit earlier this year—they are now trading around 1.5 times book, which, while not nearly as attractive as 1.2 times book, is still not near a 20-year high for the company, Rolfe noted.
Is it already too late for other investors?
Wedgewood trimmed its Berkshire Hathaway position given the stock's performance. It is still among the investment firm's top three holdings but has come down from close to 10 percent to a little under 8 percent, Rolfe said.
"While achieving book value of close to 2 times may be a thing of the past given its size, trading in the 1.5 times to 1.7 times range is not out of line," said Paul Lountzis, president of Lountzis Asset Management, a longtime investor in Berkshire.
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Rolfe said that if it traded back down to 1.3 times book value, it would be back to a 9 percent weighting.
Rolfe said many institutions have stayed away from Berkshire historically because they make investments based on sector strategies. Berkshire does not neatly fit into any sector box. "But if I'm an institutional investor and I have to put money to work ... maybe I don't care anymore that it is a conglomerate," Rolfe said. "To hell with the sector mix. ... You have to think of where we are in the stock market now, and Berkshire may be easier to swallow as a holding."