Birch's paper “Noncongruence subgroups, covers, and drawings” (pages 25–46 of The Grothendieck Theory of Dessins d'Enfants (Leila Schneps, ed.), LMS Lecture Notes Series #200) appeared in 1994, though John Voight notes a hint of it as early as a 1971 paper “Modular forms on noncongruence subgroups” by Atkin and Swinnerton-Dyer, pages 1–25 in Combinatorics (Proc. Sympos. Pure Math.) 19, bottom of p.6: “Also the number of unknowns c can be cut in half at once by observing that dj /  has factors F32 · F2.”