Yep that once endless supply of parts for the old flatbacks are getting a good bit harder to find. On the bright side at least you've got the mag box, as a lot of the tabs got broken off when folks were not removing/ installing them correctly.
You might have to try and find a whole staggered feed mag, maybe from a detachable mag, and cannibalize it. Or you could possibly find a generic mag spring or old military surplus setup and get it to work. (maybe something like this?? https://www.gunpartscorp.com/products/704590 ) Best case scenario someone here has one in a parts box they'll part with.
Good luck.
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