linux-headers-2.6.20-16-server 404 Not Found

Hello,
whenever I do "apt-get install build-essential" I got the message : "Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/l/linux-source-2.6.20/linux-headers-2.6.20-16-server_2.6.20-16.28_i386.deb 404 Not Found"!

I'm running Ubuntu 7.04 - kernel 2.6.20-16-server. On the Ubuntu-Package-Server it gives a meta-package called "linux-headers-server (2.6.20.16.28.1) [security]" but that shows to "linux-headers-2.6.20-16-server" this includes "linux-headers-2.6.20-16-server (2.6.20-16.29) [security]". You see I need the version 2.6.20-16.28 and can only get version 2.6.20-16.29.

What can I do? Please help.

regards Axel