Barclay has no regular tartan. Just dress and hunting. The hunting seems to be the original tartan from the 1842 VS. The text below is from the tartan register description for each tartan. They seem to contradict one another as to which is most commonly used by the clan.

Dress:
"Based on the earlier hunting sett which appeared in the Vestiarium Scoticum in 1842. Barclay's appear to have no 'regular' tartan. The dress version assumes this role and is the sett most commonly associated with the name."

Hunting:
"The Barclays come from Aberdeen and since that was not in the Highlands, they would not have had a tartan. However the Sobieski Stewarts 'invented' one for them in their 1842 Vestiarium Scoticum and it took hold as it was then included in J Claude's 1880 pattern books 'Clans Originaux' where it is simply labelled 'Barclay' - no mention of 'Hunting' at all. There are two versions of Barclay family tartan - one with a yellow ground and this hunting version which has a green ground. The latter comes from Vestiarium Scoticum. There is no 'regular' Barclay so the hunting is most associated with the clan."

From my experience the dress tartan is the more popular of the two for clan members. My guess is because it stands out in a crowd better than the hunting version. It is the one most used in flags, banners, and kilts.

Not sure how this fits in your theory, David. I'd be curious to know where the dress tartan originated since it came after 1842.