Yep, I heard -- clan tartans were only defined and codified in the 19th century. Plaid, however, seems to be ancient and (as you told me) a marker for Celtic presence as far as the Altai...

That's why I finally decided not to shell out for an Urquhart tartan (it would have had to be 'ancient', too, to look tolerable with the costume), but to pick out the next best thing at some brick-and-mortar shop. I'm quite content with that one.