The website linked by Jim Dixon dates the Harper's Weekly text (by "L.C.M.") to "Feb. 7, 1863," making it earlier by about four months than its appearance in teh Pittsfield Sun.

A quick look at Timothy Regan's recently published Diary, also linked by Jim, makes it clear that Regan (or his editors) interpolated later material, so that a date of "December, 1862" for Regan's text of the poem is probably insupportable. Evidence: The poem "A Child's Questions," by Horatio Alger, Jr., appears at the end of May, 1861; but it did not see print until 1863.