Yes, after submitting my previous post I did wonder whether the birth date 1898 might be correct after all. Still, Frank Brockett’s Shepherd’s Even-Song,
reportedly by the same composer as The Mocking Bird, was published by Hawkes & Son in
1901, and is mentioned on page 9 of The Fort Wayne [Indiana] News for 16 January 1904. Other first editions of Frank Brockett scores known to Worldcat include Tess (London: Francis, Day & Hunter, 1907) and The Gollywog (London: Francis, Day & Hunter in 1909). Indeed Worldcat gives the date of The Linnet (London: Francis, Day & Hunter) as 1890, although that date shouldn’t be trusted, as it apparently isn’t printed on the edition itself.
But I have no specialist knowledge of this point.