i doubt that you require an engineers certificate to MOUNT a tow bar - so long as the tow bar has a compliance plate actually fitted to itself.
As for the bar brackets fitting the different year models ... are BOTH year models JAP built? if so, the chance is really good. If its built in different countries, then probably very little chance that it will fit.
Navara D40 RX - Silver (Thai) VNOG Blog
should be rated to 2800kg or 3000kg. if its jap built and 2008 is jap built it should fit right on. when i purchased car i had a jap built towbar fitted to my thai built d22. YES THEY DO FIT but the towbar sits in 100mm cause of the tray being longer. nissan replaced it free of charge cause the stuffed up. even a year later after the towbar was installed hehe