I believe it's just SWB vehicles that were fitted with the two extra plastic underguards (I have them too). I don't know if they could be adapted to fit LWBs, or, indeed, why they weren't fitted to LWBs in the first place.
They protect the gearbox/transfer box area from light impacts and - cruciually - keep mud and road salt from the front part of the chassis, exhaust, front prop etc. They're pretty hefty things with heavy duty bolt fixings, a different order of protection from the flimsy plastic underguards on normal passenger cars