The answer lies within Jerry's excellent research. The primer in our original brass shot shell casings was .209". When we went to paper shot shells, that same primer was then case in a flanged cup. Rick's dimension drawing gives the outside dimensions, but does't show that it is actually made up of the inner, original .209" diameter primer, encased in the jacket.
I have no documentation that says the "209" was named as such, but some things are just too much of a coincidence to be otherwise.
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