It's hard to tell, but my bet is an M54 with an upgraded battery box-- based on the fender mounted air filter on the right fender. The 800 series cargo truck has the filter on the left fender. The ROKs are famous for modifying older equipment to newer standards. I'd think it would be a lot easier to put in a new battery box on an M54 than it would be to put in new ducting and a new hood on an M800 series--and why would you do that anyway? On the other hand, it could be that there's something we're not seeing on the left fender which has caused such a move. I think this is a one-off demo of a system developed for the ROK Marines in the early 80s. When I was there in 2000-02, the ROKA still had many M54s on the road.
VR, Russ