American Express is, of course, a credit card company, in the same vein as Mastercard and Visa. Except the comparison isn't a great one. Visa and Mastercard operate card-based payment networks meant to serve card issuers like banks or merchants. American Express is a payment middleman as well as the issuer.

Specific regulatory filings, along with the Oracle of Omaha's investing habits, help narrow down which secret stock Buffett may be purchasing.