So it is with Warren Buffett, whose Berkshire Hathaway (BRKA) spent $54 million in the first quarter buying shares of the No. 2 U.S. card payment-network operator, Mastercard (MA). The purchase could mark the Berkshire stock-buying debut of Todd Combs, the company’s first new portfolio manager in years.

Berkshire bought 216,000 shares of Mastercard at an average price of around $252 apiece, the company said Monday in a Securities and Exchange Commission filing.

Buffett has made a mistake or two in the credit card business, as he noted in last year’s annual report. He once tried to offer a Geico credit card in a debacle that cost Berkshire $50 million, all told.

But so far, so good on this wager: Mastercard shares, up 31% over the past year (see chart, right), closed at $279 Monday before advancing to $281 in after-hours trading. That means Berkshire is up almost $6 million on its modest Mastercard bet.

The market rally has lifted the value of the rest of the portfolio as well. Berkshire’s listed stocks were valued $53.6 billion at March 31, the company said, up $1 billion from Dec. 31.

The Mastercard purchase is the only new stock that appears in the quarterly listing of Berkshire’s holdings. None of the company’s other stock positions, led by big stakes in giant companies such as Coca-Cola (KO) and Procter & Gamble (PG), changed materially between year-end and March 31.

That marks a change from the previous quarterly report, which showed numerous changes as Geico investment chief Lou Simpson sold shares ahead of his retirement.

Buffett watchers have been wondering when the influence of Berkshire’s new portfolio manager, Combs, would start showing up in these quarterly reports. Monday’s filing may offer a glimpse, but it offers few clues as to Combs’ style. Those will have to come over time.

[cnnmoney-video vid=video/news/2011/05/16/f_berkshire_faithful.fortune/]