Originally Posted by
LIH Prem
...it found an account for me (though with the wrong number, I suspect it's the old pre-MC DC#)
More likely it is your account number, which is not the same as your card number. If you login to dinersclubus.com and navigate to Account Management > Transaction Activity you can see the last four digits: Summary Number XXXX XXXX XXXX 1234, and you can see the full number on your paper statement.