There's an article on this:Wortley, R. (1974). "'A Penny for the Plough Boys'." English Dance and Song 36: 23. (reprinted in R. Wortley, "Russell Wortley [memorial booklet]". Cambridge Morris Men (1980), 47 pp.)
(Info from: Morris Dancing FAQ Sheet: Bibliography of Journal Articles; Morris Dance Folklore Book List; and An Introductory Bibliography on Morris Dancing)
~Masato