Putting my "manufacturing engineering" hat on...
I think that it would also make sense from a production point of view to pre-stamp the magazine bottom (only) prior to assembly into a magazine tube. It would be a simpler operation, require a very basic fixture, and could even have been automated to some extent. Compression against a solid block of aluminum is also less risky to a small extent.
That way, if you spoiled something, you wouldn't be wasting an entire magazine, or the rework labor...
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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