"The drawing never called for dates on magazines. It is somebody's dream."

BR is reknowned for his knowledge and collections of sub-contractor contracts for all things Carbine, but if this is a very early 'one-off' magazine, it would have possibly been pre-drawing; the drawing may have been developed after a successful magazine was provided, and the dates would have been to identify various submittals with the revisions from the previous ones. This would have been needed to keep everything straight during the hurry-up atmosphere in post-Pearl Harbor US industry. I don't see how the floorplate is retained, but I'll take the rangemaster's idea as good. That feature would have been my first revision on that submittal, to make it be retained by the folded tabs on the body which looks more secure to me.
The font of the date stamp is a strike against this idea of a US-made prototype because it looks like a European die set, not typical of a set in use in the USAicon in 1942. A '1' for example would have been just a straight line or with serifs like the one I just typed, not with the Euro-type long angled 'flag'. I don't believe you will find a '1' like that in the numbering on any U.S. small arm of any period, particularly not WWII or earlier.
Just speculations on a slow night! We might never know what it is.