In Rob Roy, there were at LEAST 2 versions of his "kilt."
In a couple scenes it is blatantly a real belted plaid (like the "swimming scene" in the beginning).
In others it is obviously 2 pieces (like in the sword fight in the end).
In MOST scenes it is more ambiguous.

In a film like Rob Roy, more than one costume are made, to account for wear and tear on set and location. I am CONVINCED (I need to ask Peter MacDonald of I am correct) that they had BOTH belted plaids and philabegs on set for Rob's outfit.