I recall the reference now. It is in Bronson's Preface to volume II (1962, pp. x-xi). Bronson explains that he has not sought to include material from recently-published works because he does not wish to risk compromising their sales, and would in any case "not have felt free to request such a favor", even had books such as Davis' More Traditional Ballads of Virginia and Flanders' Ancient Ballads Traditionally Sung in New England appeared in time for use.

He goes on to state that, where possible, he has included lists of variants not included. He adds: "There are, besides, precisely six genuine ballad-variants for which permission was denied, which have been cued in alphabetically; those may be left for the curious to identify. I have not, however, felt obliged to take account of the versions of J. J. Niles in this way."