Bona fides: masters degree in history, minor in archeology.

I learned that as a general rule (especially in the ancient, dark age, medieval periods), by the time some literate persons got around to writing about some aspect of a remote, less-literate culture, that aspect had probably been in place for some time. And what we have as "surviving" records represent only a fraction of what was actually generated at the time.

The Gaelic culture of the Scottish highlands pre-1600 was primarily a non-literary, oral-tradition culture. This means they weren't generating written documents about their own cultural characteristics, including their unique form of garb. The "surviving" documents of the period re: Highland dress are from the occasional foriegn observer.

Thus, a surviving document from the 1590's describing a belted plaid would almost certainly indicate a practice that had been in place for some length of time....