C. M. Simpson in 'The British Broadside Ballad and Its Music', 1966, notes that the earliest ballad that can be associated with the tune "The bonny, bonny broom" is "The New Broom" of the 2nd quarter of the 17th century (ZN2211 in my broadside ballad index). It doesn't have a tune direction, but the meter and text leave little doubt of it's connection to "The Broom of Cowdenknows". This agrees with earlier sources, such as Wm. Chappell's PMOT. Information from 1975, if there really is any, would be most interesting to see, but until I do, that of 1966 will be taken as the most recent reliable evidence.

The earliest copies of "The Broom of Cowdenknowes" seem to be from the early 18th century ("The merry broomfield", ZN3627, overlooked by Child at #43) on, and are in the Douce collections on the Bodley Ballads website.