Yang Jwing-ming also translates the song in his book, _Tai Chi Secrets of the Wu & Li Styles_ (YMAA, 2001, pp. 17-19), and there he also attributes it to Wu Yuxiang. In a footnote, he notes his source as the Gu Liuxin book I mentioned, but as I said, Gu does not mention who the author of the song was, so far as I can ascertain.
Can anyone else corroborate Wu Yuxiang as the author of this poem? It consists of four lines, each a pair of seven character phrases. The style and terminology certainly is compatible with other known Wu Yuxiang writings, but it doesn’t appear among the Wu Yuxiang writings in _Taijiquanpu_, for example.
In any case, Yury, it is an interesting document!