Bruce is spot on as usual. I have a 19th Century Collection which notes the tune as "An air of Ancient origin" and a tattered and undated selection from the Gow Collection which notes "Lady McIntosh" (not MacKintosh as in later publications) as adapted from an Ancient Air. But as Neil and Nathaniel Gow published 1784-1822 neither of these references are as reliable as Bruce's 1754 trace. Until new evidence is available I would bet he is giving the definitive source to date. He calls himself an "amatuer" but knows more than most so-called professional musicologists put together. But then, too many forget the origin of that word -"one who loves" his subject.