Huh?

There are a couple of things off with Sir Malcolm's letter.

<<<...Two points here. First, I would argue strongly that due to proscription, the clan adopted it in 1603 because at that time it was not regarded as a MacGregor Tartan. It is my belief that by the end of the 18th century, nearly 200 years later, it came to be regarded as such. It is a fact that my family have worn this particular tartan since the late 18th century and regarded it as MacGregor tartan. My great. great. great, great grandfather would have worn it for good traditional reasons (see below), not on a whim. The tartan is included in the Highland Society of London's collection of 1816, which lends credence to this belief.

Secondly, if the MacGregors themselves survived the proscriptions - is it so surprising that the tartan survived? Laws passed in Edinburgh and London were difficult to enforce in the Highlands and it is quite possible that in the Highlands people would have worn the tartan of their choice. The wearing of a particular tartan demonstrates a blood connection with a particular clan. Even my kinswoman, Miss Jean Rollo, who lived in Edinburgh in 1746, made it a point to wear a tartan gown in the Canongate in defiance of the law! >>>

The Act of Proscription went into effect August 12, 1747 and the repeal was effective July 1, 1782. Maybe these are typos. Maybe Sir Malcom got it wrong. Maybe the clan adopted it in anticipation of the proscription. Maybe Miss Jean Rollo was ahead of the times.