I noticed something while looking on ebay... the G518 has seen several variations of the two frame rails that hold the lunette and the front of the tub is bolted to. I'll assume (and will look further into it) that the 'less beefy' rails are early and the beefed up rails are later. There are also several variations on the type/manner of support.
First, well, as the photo say, 1941, steel, pneumatic wheel. Note the simple full length C-channel rail.
This is from the 42 TM.. it clearly shows a different frame rail and a support. The support channel is welded to the inside flange of the C shaped frame rail.
Now this is where it seems to get manufacturer specific... the inner support, at the front, is different. Here are some photos that show the different angles that the front of the support bracket are cut at.
Not this support is cut horizontally. No taper or angle.
This support bracket is has a half height angle cut into it.
This bracket is notched... this is the exact same notch as on the 1/4 ton trailer... and Bantam did make 1 ton trailers.
And the TM photo, this time a full height angle
Water/fuel trailers also had these same brackets/varieties. My guess is that it might be possible to ID the manufacturer be the support bracket, and definitely the year of the trailer by the rail details.