Ah, well spotted. I'm so stuck in the historically accuracy mode that I didn't think of looking sideways.

I have some pictures of a tartan jacket that belonged to one of the members ‘though I don’t know whether it was necessarily in the TH style. Unfortunately the buttons were removed and sold some years ago but in a portrait of the time they look like the classic steel/brass ball type.

The cut of the jacket is similar to the green one worn by the MacNab in Raeburn’s portrait.