Asking the right questions of any government department is most important. I have always found proper research is critical.... For example, if you explain that a land rover model 110 is made in many variants including hard top and soft top of which both variants meet the ADR, and these variants are available as bolt on conversion options from the manufacturer. The compliance plate may only have "Landrover military 4x4" on it alluding to the idea that a hard top or soft top is perfectly straight forwards. What is required to show an example of a stock standard variant of each type and explain that these can be converted in accordance with the manufacturer's instructions using only parts supplied as original parts by the manufacturer.
STDDIVER
That approach might sink him before he starts.
The Perentie gained compliance as a uniquely different vehicle to the Landrover 110, to be called the "Military 4x4" (or Military 6x6). While many conponent parts were shared, they were treated as two different vehicles, in the same way that a Discovery or Range Rover also shares some common parts but is a different vehicle for compliance purposes. The Td5 Remediation vehicles are Complianced as 2003 or 2004 Defenders, and as such are not regarded as the same vehicle model as the Perentie, so any variations or acceptable configurations that are approved for the Perentie do ot automatically apply to the Defender, and vice versa.
There is also the issue that the ROPS bar as fitted to the Perentie does not seem to have any surviving engineering paperwork, so while it may have been acceptable for a 1987 certification on a Perentie (or maybe it wasn't?), it will not be accepted as a 2004 model year accesory, or meet 2018 safety requirements (unless an Engineering Signatory can be found who says that it does). It must have standalone approval for fitting today to a 2004 Defender. Even if a document turns up that says it satisfies all engineering requirements on a 1987 model Perentie, that will not approve it on a different vehicle model (2004 Defender). It would be like saying that any braking or lighting option that was approved for a 1987 Commodore will be accepted for a 2004 or 2018 Commodore.