The Supplement was prepared to provide tunes for "The Psalms of David in Metre", the 1650 version of the Psalter. Sadly, both the Scottish and Irish the split leaf versions are out of print.
This Supplement provides 180 tunes for use with the Black Psalter. It includes "Sing Through the Psalms in One Year" and "Psalmody 101" which offers a variety of helps.
More information can be found at psalmsingers.com.