Install specific older version of postgresql centos
You probably want to install from the PostgreSQL YUM Repository as it has replaced both of those repositories. The repo rpm for CentOS 5 i386 is here for download. 

The difference in the version on that particular package are from changes to the repository URL, and not changes to the versions of the related packages.

The last version of PostgreSQL 8.3 (8.3.23) and related packages are available there for use but that version and its packages are no longer supported.

If you need to build a very specific version of those packages outside of the last versions in that repository, you can always checkout the pgrpms git repository and rebuild from the .spec files there.