People obsess over Warren Buffett’s moves in the stock market, analyzing every word he says in the media and regulatory filings for clues about his investments.

But even the most sharp-eyed observers could be excused for missing a disclosure he made Friday about American Express Co.

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“The law is full of all sorts of public-notice requirements that are fulfilled by small-font ads in printed publications,” said Joseph Grundfest, a professor at Stanford University and former member of the Securities and Exchange Commission. “Those requirements are rather silly in our modern day and age.”