.... I just typed "interesting" after your post with the modified Akro box.
Now I see you say it here. I don't know enough about micas to know what that might mean on this one.
I am highly persuaded by that.
This seems like it could be an example of a box which someone would be tempted to refill the box to make it more "authentic" and in that manner lose a clue to something which we might finally learn more about many years from now.
I saw a Gropper ad which was said to have come from 1939 but it was for a Gropper top.
There also happen to be Akro marbles in the ad but no apparent connection between the companies.