If you are talking about putting only the version component and not main one, this is not the way the PGDG apt repository works. The "version" component only contains the libpq package that came with the specified version, while the main component always contains the latest stable libpq plus all the other PostgreSQL related packages. When a new PostgreSQL major release become available as a beta or as a release candidate, the only way to install it is allowing the specific libpq to be installed first, getting it from the "versioned" repository.