Install older version of a package with the "@version" specifier

[Lo Peter]

Dear all,

I am trying to install r version 3.6.3, where the current version is at 4.0.2.

Accordingly to the manual at
https://guix.gnu.org/manual/en/html_node/Invoking-guix-package.html it
should be possible to specify the package version such as "guix
package -i r@3.6.2", but on my guix (installed on slackware 14.2), it
says:
guix package: error: r: package not found for version 3.6.2

What am I missing here?

Regards,
Peter