Cajunscot (Todd) wrote: “Bottom line: the Black Watch tartan would not be appropriate for a Renaissance Fair.”

I second KEArnold’s question.
While we don’t know what tartans were worn before the Black Watch came into existence, there is evidence that there were tartans of one sort or another. And since we have little no idea what they were, or what significance they had (in the few cases where artefacts have been found), I would think Black Watch is as good as any (even if it is demonstrably later than the period intended).
Regards,
Mike