Arthur Somervell's uses the following notation on his One Thousand Exercises
Is it a standard notation? What's the meaning?
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This is called the "tonic sol–fa" system. Created by a guy named John Curwen, it's not exactly a singing system like solfège, but rather a notation system.
At the risk of publicizing myself, I asked (and subsequently answered) a question on this exact distinction at What exactly is the "tonic sol–fa" system, and how is it different from solfège?