And I just wanted to add something,
When you would like to add someone to your account as a supplementary cards holders, whatever the card that the main card holder got, Amex will use the 13 first digits as a base

For example Centurion IDC is 3745 88 and then others numbers, If you add a Gold Card to this account, The Gold card won't have the usually first digits but it will use the 3745 88 and the 7 others numbers because it cames from the Centurion account, for my own experience ony the last 2 numbers changes