I don't know what source you want, but I think I have one of the fired cases around someplace. I don't have a notarized statement signed by Douglas MacArthur and Hideki Tojo, complete with an original WWII uTube video.
M1 clips were sometimes reloaded from boxed ammo, but it was uncommon. Ammo was issued in "spam cans" already loaded in bandoliers in the appropriate clips and no one tried to keep track of ejected clips or reloaded them; they were expendable. ("Excuse, please, honorable Japanese soldier, please do not shoot while I am picking up my clips; we must keep the battlefield tidy!)
Well, the soldiers certainly didn't have a variety of clips laying around. I would guess that American soldiers found out they could use the Japanese ammo simply by trying it for the heck of it, not because of some desperate need.* My own test was based simply on proving or disproving the story of the ammo; it was interesting to find that the clips would work in an M1903 as well. (They are not a perfect fit in the guide, but they work well enough for an expedient.)
*Some say it is unsafe, but I don't think safety is high on the worry list for combat troops; the job is kind of unsafe to begin with.