This metal oval belongs to a honorarium consul (console onorario), but anyway the car has got a normal Italian
plate and not a Consular Corps plate. A honorarium consul leads an organization
with his own personnel to give support to the embassy in many tasks, like the issue of visas or the renewal of
passports. It's possible to know the name of the honorarium consul and the car this tag is attached to from the small
number printed on the tag itself.
Many thanks to Alessandro Sannia and to Nicola Biscaccinati della Fonte, honorarium consul of Guatemala, who gave me this information.