The fact that "Lass of Roch Royal" is crossed with versions of "True Lover's Farewell" in Sharp-Karples #114, and in two versions of the song in Sandburg's 'American Songbag' leads me to suspect that there was a published version of the crossed ballad, and the inclusion is not simply random addition of verses from 'common stock'. I also suspect that this 'common stock' was largely that of ballad writers, rather than that of traditional singers (speculation, not fact).