Platinum first four are 3713 and I believe all Platinum are 3713....Centurion shares the 3715 prefix with other common cards such as a green costco corporate. My guess is that Amex ran out of numbers. I really think Amex has way to many products on the market and is causing confusion. A good example is a poster on this board who thought a Platinum Optima was just the same as the Platinum charge.