Most mummer's plays were collected in the late 19th/early 20th century. Because mummers traditionally blacked their faces, when the Negro Minstrel Show craze swept Britain (in the 1850s/60s?), most mummers adopted one of these songs at the end of the play. Maybe because their audience now expected that sort of song from someone with a black face. Folk processing amd thread creep aren't purely modern phenomena.
As a consequence, the songs that had been sung pre-1850 were never collected, but there are references to a few titles in some plays. We would like to resurrect them, if possible, to be able to do a more authentic (i.e. older) version of the plays.