Actually, the arrangements of "Barren Rocks" in the Scots Guards Vol 1 (p. 116) and Seaforth Highlanders (p.102) books are 4-parted. (I have both books, and have transcribed the tunes into BMW.) Most recordings only feature the first two parts because the second two parts are sooo darned complicated! I'd say probably grade 3 if not grade 2 material, depending on the association.
You are correct in that "Sweet Maid" is only 2 parts, though.
Bookmarks