Highlanders of Rob Roy's era did not carry the sgian dubh as we know it: the small knife worn in the hose top. That practice began nearly a century after his death! The sgian shown in that display case is NOT an early 18th C. piece, as far as I can see.

Rob almost certainly carried a sgian achlais, or armpit dagger, which was a more substantial weapon than the little sgian dubh, which is just a fancy utility knife....
you beat me to it Woodsheal
but this is my understanding also