CarolC - Just as it is dangerous to presume that the "first publication" date is necessarily tied to creation of a work, it is (I think) similarly dangerous to presume that "broadsides" were ever the sole means of transmission of a song. Publishing and copright not being then what they are now, publishers (maybe "printers" would be more accurate) were apt to print songs that were popular at the time. Whether they were "original works" probably wasn't a big part of the equation.