[CentOS] Version download problem?

Trying to download the 3.1 iso's of CentOS and no matter which mirror I go to, or release 3 version, I always see the 3.6 iso's.
 
URL: http://wuarchive.wustl.edu/pub/linux/distributions/centos/3.1/isos/i386/
 
Shows all of the 3.6 iso files.
 
Some reason why I can't find the older release files?
 
Thanks,
 
Darren Young
Senior UNIX Administrator
University of Chicago
Graduate School of Business
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