Yes it does seem to be a reversal of MacGregor, excepting the white which is unchanged.

It's the first time I've knowingly seen that tartan.

I knew that a pipe band wore it, because I saw it listed in the World Pipe Band Championships programme both in 2004 and 2007.

I was noticing the number of "royal" and "personal" tartans worn by the bands:

Royal Stewart, Stewart of Appin, Stewart of Bute, Hunting Stewart
Prince Charles Edward Stuart
Princess Elizabeth
Princess Margaret Rose
Earl of St Andrews
Lady Borrodale's Gift
Shearer of Rubislaw
William Murdoch