as mentioned before, there is no sign of a stamping (number + cherry blossom) on the left frame rails.
I checked the gussets in different shades of light, but also don't see any stamping unfortunately.
I did a walk around the trailer and only observed some other points which may shed some light.
You can see on the frame rails that the trailer had the original light brackets once:
On the locations where the reflecters were, there the shape of the reflectors is visible.
This shape seems of the trimmed reflectors.
I have gone through some posts, and there is not a 100% clear answer on this.
But could it be possible that when the trailer is really late war or early post was, that they mounted the trimmed reflectors?
I hope this is visible with the large pictures...