The M103 is the bare chassis (no body) for the M104 and M105 1.5 ton cargo trailers, along with a lot of specialty trailers. It also shares the same 6-lug hubs, rims and tires as the 2.5 ton 6x6 trucks. The tires on most of these trailer are 9.00x20 (some are 11.00x20). Here is a drawing of an M103 trailer.
So, what exactly do you have? Your hubs are 5-lug as found on the Dodge 3/4 ton trucks, or on the US M101 3/4 ton trailer. That means it probably has 9.00x16 tires.
The A frame and frame appear to be much wider than an M103 or even US M101 frame. The military, for strength, had the A-frame run under the leading edge of the body, and join the side frames back a couple of feet or so. Yours has the contact point at the outer front corners. The axle appears to be much wider than either the US M101 or M103. I've gone through all my references, including ammo, generator, kitchen, signal and other trailers, and do not see any chassis that looks remotely like what you have. In fact, box trailers with fenders went out of vogue after WWII, being almost universally replaced by boxes with integral wheel wells. The chassis on the trailer you're looking at doesn't appear to have much in common with this pic of an M103 chassis.
It appears to me that someone might have taken trailer parts and made up a homebrew trailer from the pieces. It superficially looks like a military trailer, and appears to be well made. Esepcially the front and rear tailgate hinges.
Regardless, it looks like a decent trailer, and if the price is right, get it. Then take it to a scale and get it weighed. It may or may not be as heavy as an M104 or M105. I suspect it is probably closer to an M101, or between and M101 and M104/105.
