NavList:

To augment both Robin and Frank's observations, if we step outside of navigation-specific usages and look at other historic usages, it further reinforces the reality that "plain" as a spelling, was acceptable for both meanings.

English is my first language, but living so much of my life in the United States I have been forced to learn a second language - American English.  I often encounter people on both sides of our mutual language barrier who argue that one version or other has the "correct" spelling of certain words.  It is always fun to look at contemporary writings at the time the two languages began their divergence - which for sake of argument we might put at about the time of the American Revolution (or War of Independence, depending on where you went to school).

So I often turn Dr. Johnson's dictionary, which not only gives us definitions, but also often addresses pronunciation and provides citations to publications where words are used in their context.

The 1773 edition covers "plain" very well.  I attach a screenshot of the first three definitions in the list.

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