Warren Buffett’s Berkshire Hathaway BRK.B, +0.07% has been one of the most successful investments in history. In 50 years the conglomerate has turned a notional $1,000 initial investment into about $8 million, according to the annual report.

And if the past is any guide — a big “if,” naturally — then this may be a buying opportunity.

Buffett’s latest annual report and letter to shareholders came out over the weekend. There is the usual attention paid to Buffett’s various Yoda-like pronouncements about the economy and the world. But buried inside is another gem. Even though the stock tumbled sharply in price last year, Berkshire notes, the underlying value of the company rose.

Per-share net asset value jumped a healthy 6.4% in 2015, according to Berkshire, even while the shares fell by 12.5%.

“ Over the past 20 years Berkshire has fallen below this [book value] threshold very rarely. And each time turned out to be a bargain buying opportunity. ”

Today Berkshire’s stock trades at only 1.3 times book value, well below historic norms. And it’s less than twice the tangible book value per share, according to FactSet. (Tangible book value means the net value of all its investments, minus all liabilities and bogus accounting tricks.)

In modern times that has been something of a “buy” signal. Berkshire’s stock typically trades for more than twice its tangible book value — often for much more.

Over the past 20 years Berkshire has fallen below that threshold rarely. And each time it turned out to be a bargain opportunity.

Those occasions included the dot-com bubble, when no one wanted Warren Buffett’s old-fashioned investing philosophy; the financial crisis of 2008-2009; and the Europe-inspired market turmoil of 2011-2012.

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Yet over that period, Berkshire’s stock has ultimately nearly quadrupled in value. It’s almost doubled since the fall of 2011.

The past is not the future, of course, and there are no guarantees. Warren Buffett is now 85 years old and his vice-chairman, Charlie Munger, is 92. Investors may question how long the magic will continue.

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But a research paper published a little more than two years ago found that the real “magic” of Berkshire was pretty simple. Throughout its history it has basically bought high-quality stocks with leverage. Berkshire’s giant insurance operations — including, most famously, Geico — produce cheap upfront cash flow from insurance premiums. The company then invests those funds in dependable, high-quality companies such as Coca-Cola KO, -0.19% and Procter & Gamble PG, -0.10% . It’s not always easy to do. Buffett showed steady nerves during the financial crisis, for example — but it is relatively simple in theory.

Berkshire today is a giant diversified investment fund, with operations ranging from banking to insurance to mobile homes to candy to razor blades. The stock, currently trading at around 1.8 times tangible book value, may be more interesting than the chairman’s latest letter.