Apparently Mrs. McCord herself added the "Lady Mary" in later renditions of the song (including Max Hunter recording, 1960). The version I posted is from the 4-vol. set.
I would guess that the comment about it possibly being from a sheet music song of the 1880s or 1890s was added to the abridged version by the editor, Norm Cohen; it is not in the 4 vol. set.
Not important, but after listening to the Max Hunter recording of Mrs. McCord, I think "flower-strewn bed" probably was there from the start rather that "flower-strewn bier." The lyrics to the Max Hunter recording have two errors; line 1 should be "came from" and in line 3 "his look" ends the sentence.

We have not found out where Sandburg published the song (see Malcolm's post). He published a great deal, so a reference is needed.