To display the DIAG version installed in the DIAG OS, use the dpkg -l | grep dn-diags command at the root@dell-diag-os:~ prompt.
root@dellemc-diag-os:~# dpkg -l | grep dn-diags
ii dn-diags-<platform>-on.deb 3.xx.4.1-x amd64 Dell Networking Diagnostics