Anatomy of Amex card numbers code
The Amex card number is made of 5 fields, the first 2 digits are always 37 :

37AA-BBBBBB-BCDDE

long time ago, the AA field was used to code the base currency of the
account outside of the US (and thus the country). This changed a bit
after credit cards (Optima cards) were launched in the 80's, they
became their own AA value. In the US, many different values for AA
exists also for charge cards.

AA field : base currency code (usually), many different values used in the US

BBBBBBB field : "account number" , with different variants. Centurion
or older Platinum accounts usually begin with 88 etc.

C field : card sequence number. Begins with 1, become incremented each
time a new card number has to be assigned (after a loss/theft of the
card for instance).

DD field : "00" for the main card, "01" for the first supplementary card, etc.

E field : Check digit, computed using the LUHN-10 algorithm.