Here is the note as printed by Tolman and Eddy in the Journal of American Folk-Lore, Vol. 35 (1922). They are quite right that the "Answer" really exists as two separate ballads that tell a very similar story. Likely a hack for some Broadside printer read the first but did not buy a copy, then re-wrote the ballad himself from what he remembered. Also, from this we learn that the four-line happy ending occurs in Grieg as well, so it was around in Scottish oral tradition. Okay, here's the note:

There are two songs which go by the name of "The Banks of Sweet Dundee." The original song to which Professor Tolman's text belongs and his references apply and which is also known as Undaunted Mary is common in English broadsides. It runs to ten stanzas. See Harvard broadsides as follows 25242.17, vi, 149, and ix, 79 (Bebbington, Manchester, No 83); vii, 117 (Catnach); xi, 15 (Such); Pitts; George Walker, Durham, No 6. The Walker broadside has an additional stanza which appears also in Ford (as cited above) and in the text collected by Greig in Scotland "Folk Song of the North East" Ixvi For America see "De Witt's Forget Me Not Songster" p 94; "We Parted by the River Side Songster," (New York cop 1869) p 44; "Henry de Marsan's New Comic and Sentimental Singers Journal," i 37 No 5; "Irish Come All Ye's p 68; "Delaney's Scotch Song Book No l" (New York) p 3; "Wehman's Irish Song Book No I" (New York cop 1887) p 117; "Wehman Bros Pocket Size Irish Song Book No 2" (New York cop 1909) pp 6-7; Wehman broadside No 274; Andrews broadside, List 6, No 81. An imperfect copy was taken down in 1910 by Mr FC Walker in St John New Brunswick. Mackenzie pp 47-48 prints a Nova Scotian version.

The other song is a sequel or answer. This is the piece published by Christie, 1 258-259 as "The Banks of Sweet Dundee." It recounts the heroic deeds of Mary's lover who has been pressed into the navy and tells of their happy reunion. Harvard College has it in broadsides issued by John Ross (Newcastle) No 19 25242.17 iv 184, and JO Bebbington (Manchester) No 320 25242.17 x 68; and Greig gives a text Folk Song of the North East xxx. A somewhat different answer telling the same story is in broadsides issued by Ryle & Co, 25242.17 vii 238 and C. Paul I have no record of the printing of either answer in America.