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Funny thing is it does have a 4 stamped into it front and back.
The "4" has nothing to do with the Number 4 rifle. It means that this was the fourth version of the Number 1 magazine. Another indication of the Nunber 1 magazine is the side ribs of the magazine itself. In the Number 1 Magazine, these indented ribs run all the way to the bottom, while in the Number 4 magazine, the ribs stop about 1/4 of an inch or so from the bottom.