Next question: if Mrs Brown wasn't the inventor of the unpaid mason motif, was it Herd? Obviously it could have been, but given that the opening stanza first appears in the fragment of the ballad in his manuscript and not just in the fuller text he published in 1776 (much of which I'm sure Child was right to regard as "spurious"), I would be inclined to give him the benefit of the doubt and accept that a version in which Lambkin is a mason with a grievance at not being paid was probably already in circulation in Scotland by the 1770s.

That means that Mrs Brown (or Anna Gordon as she would have been then) could well have known it long before she saw Herd's book. (Alexander Fraser Tytler sent her a copy of Herd in April 1800, and her letter to him in December suggests that hadn't been familiar with the book before that -- the letters are on pp. 36 and 49 of Rieuwerts's book). On the other hand it is perhaps a little odd that she didn't produce her version of Lamkin until after she had seen Herd's version -- although I like David Fowler's comment that even if she did use Herd's text in putting together her own, she "managed to turn lead into gold".