MCO = Miscellaneous Charges Order. Used both for ticket refunds, and by travel agents when setting up tickets on departure.

Have been sent MCOs on a number of occasions. They're usuall valid a year (though the last one I had - from FlyBE - was only valid six months), so if you're planning to use that airline again they're worth having. If you're not planning to use that airline the trick to use (so I am told) is to use the MCO to buy a fully-refundable ticket and then get a refund on that (they may give you another MCO, but at least you will then have extended the validity)

If you're worried about them trying to refund your old Amex account just make sure you send a suitable covering letter when you return the ticket.