Fuel ,water or oil ?
Until I can find more research material on this as a fuel wagon,I think that I shall just call it a tanker wagon of some sort ,as it could have been used also for oil ,water or other liquid.
The problem I have with it as a fuel wagon is that I don't know how or where a pump and hose would be connected.If used for water gravity feed would be enough provided by the tap at the rear of the tank but if used for fuel a hand operated wobble pump of some kind would have to be installed but where?Then there is the hose to consider, where and how would it be properly stowed?
This same wagon could be built in any scale,horse drawn or whatever.Just find a nice set of wheels,chassis and springs and use your imagination from there.A lot of this ground equipment was actually built this way.A car maker would supply the chassis or a beefed up version thereof, and the rest would be constructed as need be.Even on military fields I have seen a wide variety of stuff that was adapted for "in the field" use.For a semi-scratchbuilder like me that suits me just fine and is one of the big reasons that I do WW1 dioramas, as it gives me lots of room for artistic license.