Hello,
I too have a trailer of this style I bought years ago to go with my M7 Gen Set. Originally thought this was an M7 but after looking close over the years, I don't think this is.
Have yet to decide which model this one is. My hitch frame is hinged to facilitate the tilt mode and does not match the photo of the M7 in the book.
Does not have nor does it have any indication of the structure of the Director trailer style. Has the verticle shaft with the small wheel on top the tongue like yours, but the wheel is broke --just the center is still there. Does look like there was a place for the winch on the "deck" in the front.
All of the corner jacks are missing but there are plenty of clues left that tell me it was equipped with them. This has been beat up pretty good over the years, a contractor had it to move a Skid Steer around with it and was NOT gentle.
Look mostly like an M18, but still not everything matches.
I have a copy of TM9-881. It is dated 22 DEC 1944. It actually does not show any listing or info for a M1 .
Mine is still buried under three feet of snow so can't get pics right now, but hey it's the same shade of yellow!
Good luck with yours,
Jim