I've been wrong before, but that photo sure does look like the 400 gallon "water buffalo" trailer. That long box on the left front would have 5 (I think) spigots for filling canteens. In original WWII fit it would have sockets for 4 bows to carry a trap to match the profile of the similar 3/4 ton trailer.
To, OP, there are some photos of the 600gallon dual-tire fuel trailer online. The Army Air services seem to have called them "A-3" which may help your search. The A-3 trailer may have been fitted for 5 bows like the 1-1/2 ton traier. Or not.