Amex cards extra four digits

[Greg_Rose@sibelius.sydney.sterling.com (Greg ROSE)]

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Oops, following up my own posting, it proves I
should have been more careful...

I wrote
  Wow, you learn something every day. I've never in
  my whole life (well, my American Express life,
  over 15 years) noticed those four digits. I've
  never been asked for them by anyone, or noticed
  them being written down, either. Exactly when and
  how are they used?

But when I reread it I thought it sounded
sarcastic. It isn't -- there really are an extra
four digits printed on both my Amex cards that I'd
never noticed before -- and my question is
serious.

Greg.

Greg Rose               INTERNET: greg_rose@sydney.sterling.com  
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NSW 2086 Australia.     co-mod sci.crypt.research, USENIX Director.

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