If the CG says 3 seats that’s what it will be insured for… surely… max 3 seats/people
… seems fair enough
Our camping car is a 3 berth and the carte grise states 4 seats which is correct. However there are only 2 seatbelts. There is no way we would carry any unrestrained passenger it would be far too dangerous.
is there a reason why you can’t just fit the extra belts??
we fitted 'em to vehicles which (quite legally) had none…as we considered it safer to have the belts for us and our passengers… 
It would be difficult as they are like little side facing bench seats. Fortunately there are only two of us so it should never be a problem. It is just odd - as if the vehicle wasn’t properly specified or checked when it was first registered.
that does sound odd… and, as you say, better left unbelted and thus unused… in transit.
what age is it… and when registered in France?
It was registered new in France in December 2003. As far as I know all vehicles of that model were registered with 4 seats on the carte grise.
Well it has seating installed from new for up to 9
Was the vehicle built in France???
it depends on whether or not the seating installation meets French standards… I would think…
there is often a hiccup between countries… when standards are not the same
Land Rover vehicles are not built in France. The EU compliance has always been certified in France via the French sales organisation, Landrover.fr
I need to recheck the details I got from Landrover heritage when I bought it, but suspect that does not mention how many seats were fitted ex factory, nor usually does the UK v5 document either. (The documents are in France and I am currently in Wales).