According to the tartan notes, the tartan is restricted and may only be worn by those of the name Mar and those granted permission to wear it by the Countess of Mar. That's different from a clan chief adopting an existing tartan to represent his clan while leaving its status otherwise unchanged. Members of the Royal Regiment of Scotland don't need the Duke of Argyll's permission to wear their tartan. Of course, I guess you could say the Red Mar tartan was an "orphan" until the Countess of Mar adopted it, but I don't see how the Countess of Mar can claim rights akin to copyright on the pattern, when it was created over 250 years ago by an unknown weaver.

Regarding the cloth, it seems rather ordinary to me. The colors resemble other 18th-century tartans. I see folds and a few flaws, the aforementioned initials, and a hem on the top (according to the orientation of the pictures) sewn with red thread. On the right is a rather narrow red herringbone selvage. I'm guessing the cloth has been cut on the right side. If the right side was also a selvage, that would be unusual, as there is no herringbone pattern, and the weft pattern isn't centered. However, the dimensions given in the tartan notes would suggest that this was just one half of a plaid. Or perhaps a philabeg, as it isn't attached to the other half.