A rather lengthy reply was unfortunately swallowed into the ether, so here is a more abridged version of what I was trying to post.

The 1704 letter has been oft-cited as "proof" that clan tartans eixsted prior to the generally accepted date. In reality, all this letter tells us that the cheif of the Grants expected his men to turn out in "red and green" tartan on the field. It does not specify a particular sett or design. In fact, if it proves anything, it proves that clan tartans were not in use, for if they were, all the cheif would have had to have done is to instruct them to wear "the proper Grant tartan" and no more would have been neccesary.

As for the Richard Waitt protraits of 1714, here they are:



Yes, these two figures are wearing similar tartans. But again, what does that tell us about "clan tartans?" All we can deduce from this is that two men in the same household were wear clothing seemingly made from the same cloth. Is this all that unusual? Does this mean it was a "clan Grant" tartan? No. In fact, if it proves anything, it proves too much, for this tartan is nothing like the Clan Grant tartans of today.

I note that the Clan Grant USA, in the "history" section of their web site, again claim that the Black Watch regiment took their tartan from a pre-existing Grant tartan. They write, "the Clan supposedly has the reciepts for the original order of the tartan material."

Well, I'd like to see that documented. Saying that the clan "supposedly" has such material proves nothing. Show me the evidence so that I can look at it ojectively and see what it really tells us. If it proves what they claim it does, it would cause us to rewrite tartan history. This means that any evidence needs to be looked at critically and subject to peer review before any wild claims are made. In other words, we need to proceed with caution.

I will end by quoting what the UK Clan Grant has to say on their site:
http://www.clangrant.org/tartan.php
An argument has been put forward for the Black Watch tartan having originated as the Clan Campbell tartan because of the large number of Campbells serving in its ranks. In fact the reverse is almost certainly true: the regimental tartan was adopted by the Campbells as theirs because so many Campbells were already accustomed to wearing it when the idea of wearing clan tartans became general. This explanation would account for its use by the other clans mentioned [Grant, Munro], all of whom were involved in the formation of the Black Watch as well as the Campbells.