Hi Guys,
The trailer in the pic is either an Australian built #3 or #3A. It could have been built by Ford, GMH, Charles Hope or International Harvester (and another company i think whose name eludes me!). Don/Tipdog is right- look at the loication of the guards/fenders- the #4 has them mounted higher- this is the easy give away for the identify of model #.
The #1 and #2 were nothing like a standard jeep trailer as we know them to look. Low straight sides, tailgate, open wheels-no guards, centre pole drawbar with looped round stock welded onto the pole to form the towing eye (modified #2's had a sprung lunette eye fitted), no lights, no reflectors. Very basic but quite beautiful (arent the Jeff

)!
The #3 was copied from the MBT/T3 hence the simliar look- solid waterproof/floatable tub with screw in drain plugs in the floor, angled/flat guards welded on the vertical side of the tub, lunette on top of the drawbar (located in a very simplified version of the casting!- a piece of round tube with a bronze bushing!), different pressings in tub, solid tube under the rolled lip around the top for strengthening, dataplate mounted on the rear in centre, square axle shaft, 2 helpers per spring pack, small 1' round reflectors mounted in pairs vertically or horizontally depending on manufacturer, round tube 2 piece landing leg with locking pin to hold in position and another to raise lower the height), no lights factory fitted.
The #3a was then a modified version of the #3- same as the #3 but included combats, brake drums (but no pads!) and some fittings for a handbrake but it was never fitted, bar tread tyres.
The #4 was then a lightened version of the #3/3A. Angled/flat guards welded on the angled lip of the tub side. Drain pugs in floor and rear panel, lunette moved into the drawbar (as a method of lowering the traveling height and angle of the #3/#3A when behind a jeep- the #3/#a leant forward causing loads to shift- some #3/#3A's were modified in field as a work order until the #4's came out in 1945), no handbrake provisions, no brake drums, round axle, similar landing leg to the MBT/T3, very light weight compared to a #3/#3A, MBT/T3, rolled lip on tub- no strenghtening tube, not a very strong chassis (if you can call it that!), simple drawbar, no reflectors, no lights factory fitted, solid rims (similar to a slat/prototype rim), highway tread/road tyres.
Hope this helps,
Ian.
(edited when i had more time to explain myself)