My use of the word 'writing' in my 07 post above was a bit strong and misleading.

Martin's actual note reads: 'He [Stevenson] was at a low ebb of his life and even contemplating suicide at this time. I like sad songs and it seems a perfect combination: suicide and the name Susquehannah. So I gathered some verses he had written about it, changed the words round a bit, but still used most of the same letters he did (but I did it my way). The tune just seemed to appear as if it was intended for it'.

--Stewie.