Here is a thought. Some of these parts are constructed of components that are available at the time.
Presuming that the "wrench flats" are there because the next threads on most likely is a misconception.
You can "weld" aluminum very easily now with the advent of "lumaloy".
It's works just like traditional lead solder without needing the flux.
I would not presume that is not what the "fabricator" did on that tank?
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