What you have looks to be a military trailer that has received a lot of previous owner mods. Aside from the coupler change, it appears to have commercial trailer fenders, different spring mounts (Toyota maybe?), some boxing in at the rear, no landing leg, etc..
I believe the bottom width of the tub is the quick and dirty tell for M416 vs earlier 1/4 ton trailers. They are wider but I don't recall the difference. 42" vs 38" or something like that. Handbrake handles are also different early to late. I can't tell exactly from the photo but it sort of looks like the remnants of an M100 or MBT handle is there; not sure.
Is there a tailgate in it? That might explain the boxing at the back.
I would actually prefer having one like this - make me feel a lot better that I'm not cutting up an unmolested original. Looks like a fun project and it has the right "patina" for an off roader.
dig it.