My mother was born in 1910 near Bilston in the Black Country and remembered people coming round selling song sheets, then singing them so you could learn the tune. No wonder there so many variants nationwide!.
Even I (b 1944) living on edge of Wolverhampton - remember things now long gone, a man on a bike selling wooden line props 'Line props, get yer line props' our milkman, small van,calling 'Milk-o lovely milk-o' Milk in a churn and you took out something to put the milk in and he measured it out with a dip can. Bread man with horse, later small van and Scrap collectors. .Calling out 'Scraaap iron any scraaap iron' In fact we still have one in Telford.. but small lorry with recorded sounds from speaker om roof

Many of the musicians could read music..and write arrangements. .See Village Music Project for more info.
Many latterly also had collections of 78s from which they learned tunes and songs...then altered them to fit their singing style. Prime example is a Boer War song 'The Baby's Name' but I'm damned if i can remember the mans name - but he had a version which was a VERY garbled form of the original!